Electron beam apparatus and device production method using the electron beam apparatus

ABSTRACT

The purpose of the invention is to provide an improved electron beam apparatus with improvements in throughput, accuracy, etc. One of the characterizing features of the electron beam apparatus of the present invention is that it has a plurality of optical systems, each of which comprises a primary electron optical system for scanning and irradiating a sample with a plurality of primary electron beams; a detector device for detecting a plurality of secondary beams emitted by irradiating the sample with the primary electron beams; and a secondary electron optical system for guiding the secondary electron beams from the sample to the detector device; all configured so that the plurality of optical systems scan different regions of the sample with their primary electron beams, and detect the respective secondary electron beams emitted from each of the respective regions. This is what makes higher throughput possible. To provide high accuracy, the apparatus is configured such that the axes of its optical systems can be aligned, and aberrations corrected, by a variety of methods.

This application is a divisional of application Ser. No. 09/985,323filed Nov. 2, 2001.

BACKGROUND OF THE INVENTION

The present invention is related to an apparatus for using a pluralityof electron beams to inspect for defects, etc., in patterns formed onthe surface of a sample; and in particular, to an apparatus forperforming high-throughput wafer defect detection (such as, for example,in a semiconductor device fabrication process) by irradiating the samplewith an electron beam; detecting secondary electrons (which varyaccording to the properties of the surface of the sample); forming imagedata therefrom; and inspecting and evaluating patterns, etc., formed onthe surface of the sample, based on that image data. It is also relatedto a device fabrication method for high yield production ofsemiconductor devices, using such an apparatus.

Conventional systems using scanning electron microscopes (SEM) arecurrently available for performing the above wafer inspection function.These systems form an SEM image by raster-scanning a finely focusedelectron beam over an extremely closely spaced raster width, whiledetecting secondary electrons emitted from the sample using a secondaryelectron detector. Defects are then found by comparing the SEM imagewith a reference standard image.

Because of the small beam size, pixel size, and raster width used in SEMsystems adapted for defect inspection, however, such inspectionsrequired a huge amount of time. Also, when the sample was irradiatedwith a larger beam to improve throughput, this produced degraded spacialresolution quality of the SEM images.

Multibeam inspection systems, in which the sample is irradiated with anumber of beams at the same time, have also been in development over thepast few years. Many improvements to such systems are needed, however,for realization of high throughput, while also maintaining goodprecision.

SUMMARY OF THE INVENTION

It is therefore the basic objective of the present invention to makeimprovements to prior multibeam scanning systems, and more specifically,to provide better throughput.

To achieve this objective, the present invention provides an electronbeam inspection apparatus capable of higher throughput. The apparatuscomprises a plurality of primary electron optical systems for directingprimary electron beams (emitted by an electron gun) toward a sample(such as a wafer); and, a plurality of secondary electron opticalsystems (columns) for guiding, to a secondary electron detector,secondary electrons emitted by irradiating the sample with the primaryelectron beams; such that a different region of the sample is inspectedby each of the optical systems.

Also, to improve the accuracy of inspections of samples performed usingelectron beams, the present invention provides an electron beaminspection apparatus configured to provide more precise axial alignmentof its electron optical systems. More specifically, alignment of theaxes of the multiple beams is performed by adjusting the elements of theoptical system such that when voltages applied to the elements (lenses,etc.) of the electron optical system are changed, electron beams thatare the same distance from the center of the multiple beams will exhibitsubstantially the same amount of change in position on the sample.

Also, in a another alignment method of the present invention, axialalignment is performed by detecting the center of an aperture stop atwhich the image is formed, and adjusting elements of the electronoptical system so that secondary electron beams will pass through thecenter of this aperture stop.

Also, according to one aspect of the multibeam inspection apparatus ofthe present invention, a multi-aperture plate having a plurality ofapertures is used to obtain multiple beams from a single electron gun.In this aspect, the highest intensity portions of the electron beamemitted from the electron gun are aligned with the multi-apertures suchas to obtain multibeam beamlets having high beam currents, in order toestablish the good optical conditions for performing inspections.

In addition, provided in another aspect of the multibeam inspectionapparatus of the present invention, is an electron beam inspectionapparatus in which is provided a correction device for irradiatingirradiation design points with primary electron beams, to therebyestablish good optical conditions for performing inspections.

The present invention also provides an electron beam inspectionapparatus wherein aberrations such as magnification and rotationchromatic aberration can be corrected by adjusting the position alongthe optical axis of a crossover formed by the primary electron beams.

The present invention also provides an electron beam inspectionapparatus in which shot noise is reduced, by establishing conditionssuch that the electron gun is operated in the space-charge-limitedregion of its characteristic curve.

The present invention also provides an electron beam inspectionapparatus configured for measuring the irradiation dose applied to thesample by the electron beams, and performing control actions to stopoperation of the electron beam inspection apparatus when it isdetermined that the dose is abnormal.

In addition, the present invention provides an electron beam inspectionapparatus wherein the electron optical elements such as electron lensesand deflectors that make up its optical systems are constructed suchthat, rather than using fasteners such as bolts to join the separateinsulators and conductors provided therein, conductive layers are formedon required areas of the insulators by electroplating, to thus providecompact electron optical elements of simple construction.

Also provided in the present invention, is a device production methodwherein an electron beam inspection apparatus as described above is usedto perform in-process inspection of samples such as wafers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation view showing the major components that make upthe inspection apparatus of the present embodiment, and a cross-sectionview of Section A-A of FIG. 2.

FIG. 2 is a plan view of the major components that make up theinspection apparatus of the present embodiment shown in FIG. 1, and across-section view of Section B-B of FIG. 1.

FIG. 3 is a cross-section view of Section C-C of FIG. 1, showing aminienvironment unit.

FIG. 4 is a cross-section view of Section D-D of FIG. 2, showing aloader housing.

FIG. 5(A) is an enlarged side view of a wafer rack.

FIG. 5(B) is a cross-section view of Section E-E of FIG. 5(A).

FIG. 6 (A) shows one variation of a method of supporting a waferhousing.

FIG. 6(B) shows another variation of a method of supporting a waferhousing.

FIG. 7 is a schematic diagram of the electron optical system of theinspection apparatus of FIG. 1.

FIG. 8 is a drawing to show the positional relationships of theapertures of a multi-aperture plate used in the primary optical systemof the electron optical system of FIG. 7.

FIG. 9 shows an electrical potential application system.

FIG. 10(A) shows a side view of an electron beam calibration mechanism.

FIG. 10(B) shows a plan view of the mechanism of FIG. 10(A).

FIG. 11 is a simplified diagram of an alignment control system forwafers.

FIG. 12 is an enlarged cross-sectional side view of a cassette holderand minienvironment unit.

FIG. 13 shows steps for performing inspections using the electronoptical system.

FIG. 14 is a cross-sectional plan view of a ExB separator.

FIG. 15 is a cross-sectional side view of a ExB separator.

FIG. 16 shows how a sample (wafer) is scanned during irradiation bymultiple primary beamlets.

FIG. 17(A) shows a front view of the vacuum chamber and XY stage of aprior electron beam inspection apparatus.

FIG. 17(B) shows a side view of the vacuum chamber and XY stage of FIG.17.

FIG. 18 shows a differential discharge device used in the XY stage ofFIG. 17.

FIG. 19 shows the vacuum chamber and XY stage in one embodiment of theelectron beam inspection apparatus of the present invention.

FIG. 20 shows an example of the operation of a differential dischargemechanism provided in the apparatus of FIG. 19.

FIG. 21 shows the gas circulation line system of the apparatus of FIG.19.

FIGS. 22(A) and (B), respectively, show front and side views of thevacuum chamber and XY stage in one embodiment of the electron beamapparatus of the present invention.

FIG. 23 shows the vacuum chamber and XY stage of another embodiment ofthe electron beam apparatus of the present invention.

FIG. 24 shows the vacuum chamber and XY stage of a different embodimentof the electron beam apparatus of the present invention.

FIG. 25 shows the vacuum chamber and XY stage of yet another embodimentof the electron beam apparatus of the present invention.

FIG. 26 shows the vacuum chamber and XY stage of another embodiment ofthe electron beam apparatus of the present invention.

FIGS. 27( a)) and (b), respectively, show top and side view schematicrepresentations of the optical system of one of the columns of amultiple column electron optical system of the present invention.

FIG. 28 shows a first placement configuration of multiple opticalsystems in the multiple column electron optical system of the presentinvention.

FIG. 29 shows a second placement configuration of multiple opticalsystems in the multiple column electron optical system of the presentinvention.

FIG. 30 is a drawing to aid the description of a pattern defectdetection method.

FIG. 31 is a drawing to aid the description of a line width measurementmethod.

FIG. 32 is a drawing to aid the description of an electrical potentialcontrast measurement method.

FIG. 33 is a drawing to aid the description of an electron opticalsystem axial alignment method.

FIG. 34 shows alignment marks provided on a sample used in the alignmentmethod of FIG. 33.

FIG. 35 is a diagram (of an electron optical system) to aid thedescription of the alignment of a secondary optical system.

FIG. 36( a) shows the positional relationship between an aperture stopimage and addresses at completion of axial alignment of an opticalsystem.

FIG. 36( b) shows the positional relationship between an aperture stopimage and addresses during axial alignment of an optical system.

FIG. 37 shows the optical system of a typical electron beam apparatus.The drawing is provided to aid the description of axial alignment withthe axis of a Wien filter (ExB separator) according to the presentinvention.

FIG. 38 is a schematic diagram of an example a main portion of anelectron beam apparatus of the present invention.

FIG. 39 shows the correspondence relationships between the electronbeamlets and the apertures of the multi-aperture plate of the apparatusof FIG. 38 prior to adjustment.

FIG. 40 shows the correspondence relationships between the electronbeamlets and the apertures of the multi-aperture plate of the apparatusof FIG. 38 after axial alignment.

FIG. 41 shows the correspondence relationships between the electronbeamlets and the apertures of the multi-aperture plate of the apparatusof FIG. 38 after adjustment according to the present invention.

FIG. 42 is a schematic diagram of a defect inspection apparatusconfiguration according to the present invention, comprising a systemfor preventing positional displacement between a reference standardimage and an image to be inspected. The diagram is also used to describecalibration of the electron beam inspection apparatus with respect todeviation between an actual irradiated point and a design point,according to the present invention.

FIG. 43 shows an example of multiple inspection images acquired by thedefect inspection apparatus of FIG. 42, and a reference standard image.

FIG. 44 is a flow chart for a main routine for performing inspection ofwafers using the defect inspection apparatus of FIG. 42.

FIG. 45 is a detailed subroutine flow diagram for the step of acquiringdata for multiple inspection images (Step 304-9) in the main routine ofFIG. 44.

FIG. 46 is a detailed subroutine flow diagram for the comparison step(Step 308-9) in the main routine of FIG. 44.

FIG. 47 shows an example of a specific configuration for the detector ofthe defect inspection apparatus of FIG. 42.

FIG. 48 is a drawing to illustrate a concept wherein the positions ofmultiple inspection regions on the surface of a semiconductor wafer areshifted such that they are offset from each other in position, butpartially overlap.

FIG. 49 is a simplified front view of a first multi-aperture plate.

FIG. 50 is a simplified plan view of a mark pad.

FIG. 51 is a flow chart for a method of calibrating the irradiationpositions of a plurality of electron beams.

FIG. 52( a) is a simplified drawing showing a method of calibrating theX-axis irradiation positions, on a mark pad, of a plurality of primaryelectron beams.

FIG. 52( b) is a simplified drawing showing a method of calibrating theY-axis irradiation positions, on a mark pad, of a plurality of primaryelectron beams.

FIG. 52( c) is a simplified drawing for describing aligning the positionof the optical axis with a mark position.

FIG. 53( a) shows the waveform of a signal output when multiple primarybeams are scanned over a mark pad along the X axis.

FIG. 53( b) shows the waveform of a signal output when multiple primarybeams are scanned over a mark pad along the Y axis.

FIG. 53( c) shows the relationship between deflection voltage and signalstrength when the irradiation positions of the primary beams areproperly calibrated.

FIG. 54 is a schematic diagram of an electron optical system, used todescribe adjustment of the position of a crossover.

FIG. 55 is a schematic diagram of an electron beam inspection apparatusoptical system, used to describe a method of reducing shot noiseaccording to the present invention.

FIG. 56 is a schematic diagram of an electron beam inspection apparatusoptical system, used to describe another method of reducing shot noiseaccording to the present invention.

FIG. 57 is a schematic diagram of the optical system of an electron beaminspection system in a multibeam inspection apparatus with a dosecontrol function incorporated therein.

FIGS. 58( a) and (b) are flow diagrams showing the operational flow inone embodiment of a sample protection mechanism.

FIG. 59 is a simplified drawing showing the construction of oneembodiment of one of the charged particle beam control elements of thepresent invention.

FIG. 60 shows a cross-section view of the charged particle beam controlelement of FIG. 59.

FIG. 61 shows a top view of a prior electrostatic deflector.

FIGS. 62( a) and (b) show cross-section views of section A-A and B-B,respectively of FIG. 61.

FIG. 63 is a flow chart showing the steps of a device fabrication methodin which the inspection apparatus of the present invention is used toperform in-process inspection of wafers.

FIG. 64 is a flow chart for the lithography step of the method of FIG.63.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A semiconductor device inspection apparatus that constitutes oneembodiment of an electron beam inspection apparatus of the presentinvention is described below, with reference to the drawings.

Overall Configuration of the Semiconductor Device Inspection Apparatus

Shown in FIG. 1 and FIG. 2 are elevation and plan views, respectively,showing the major components that make up the semiconductor inspectionapparatus 1 of the present embodiment.

The semiconductor inspection apparatus 1 of the present embodimentcomprises the following major components, disposed in relative positionsas shown in FIG. 1 and FIG. 2:

-   -   a cassette holder 10, for holding a cassette containing a        plurality of wafers;    -   a minienvironment unit 20;    -   a main housing 30 that forms a working chamber;    -   a loader housing 40 placed between the minienvironment unit 20        and the main housing 30, that forms two loading chambers 40 and        41;    -   a loader 60 (61, 63) for taking a wafer from the cassette holder        10 and loading it onto a stage 50 placed in the main housing 30;        and    -   an electron optical apparatus 70 (a system for scanning an        electron beam) installed in the main housing 30.

The semiconductor inspection apparatus 1 also comprises an opticalmicroscope 871, which further comprises

-   -   a precharge unit 81, placed in the main vacuum housing 30;    -   an electrical potential application device 83 (shown in FIG.        10), for applying an electrical potential to a wafer;    -   an electron beam calibration system 85 (shown in FIG. 10); and    -   an alignment control system 87, for positioning a wafer on the        stage 50.

The parts of the semiconductor inspection apparatus mentioned above alsoconstitute vacuum pumps, vacuum valves, vacuum gages and vacuum lines(not shown in the drawing) for evacuating the electron optical system,detector, and working chamber (to be described later), in a prescribedsequence. A vacuum valve in each section is controlled to obtain therequired level of vacuum therein. The vacuum is constantly monitored,and if an abnormal reading occurs, an interlock function activatesemergency control of an isolation valve (not shown), to seal off thechamber from the evacuation system, etc., and maintain the requiredlevel of vacuum in the various parts of the system. For the vacuumpumps, a turbomolecular pump may be used for the main vacuum, and aLutz-type dry pump may be used for rough pumping. The ambient pressurein the vicinity of a wafer on the stage (the part to be irradiated by anelectron beam) must be between 10⁻³ and 10⁻⁶ Pa, but should preferablybe between 10⁻⁴ and 10⁻⁶ Pa.

Cassette Holder

The cassette holder 10 is configured to hold a plurality of cassettes c(two, in this embodiment) each of which contains a plurality of wafers(e.g. 25) placed such as to lie flat, one over the other. The cassettesc may be closed cassettes such as the SMIF (Standard MechanicalInterFace), or FOUP (Front Opening Unified Pod) cassettes manufacturedby Asyst Technologies. Either an automatically loaded or manually loadedcassette holder may be installed, at the option of the user. That is, ifthe cassettes are to be transported to the apparatus and loadedautomatically by a robot, a closed cassette holder configured forautomatic loading is installed; and if cassettes are to be loaded byhand, an open cassette holder designed for manual loading is installed.In this embodiment, the cassette holder 10, which is designed forautomatic loading of cassettes c, comprises, for example, an elevatortable 11, and an elevator mechanism 12 for raising and lowering theelevator table 11. A cassette c is first loaded on the elevator table inthe position shown by dotted lines in FIG. 2. It is then automaticallyrotated by a first transporter unit 61 of the minienvironment unit (tobe described later), to the position shown by solid lines, where it willbe possible to remove wafers from it. It is then lowered on the elevatortable 11 to the position marked by dotted lines in FIG. 1. Because themany wafers contained in the cassette c are placed to lie flat, one overthe other, with separation therebetween, the arm of the firsttransporter unit is configured for being moved up and down, so that thetransporter will be able to access any wafer in the cassette. FIG. 12shows the relationship between the first transporter unit 61 and aloaded cassette c.

Substrates (wafers) to be inspected for defects are loaded in a cassettec. This inspection may be performed either during or after the waferprocessing step of the semiconductor device manufacturing process. Morespecifically, the cassettes c could contain substrates (wafers) thathave gone through processes such as etching and membrane-forming(including copper plating), CMP (chemical-mechanical polishing);planarization, or ion implantation; substrates (wafers) that have haddevice patterns formed on their surface; or wafers on which devicepatterns have not yet been formed.

Minienvironment Unit

As shown in FIGS. 1-3, the minienvironment unit 20 comprises

-   -   a housing 22 forming a minienvironment space 21 such that the        atmosphere therein can be controlled;    -   a gas circulation system 23 (231-233) for circulating a gas,        such as clean air, through the minienvironment space 21 for        controlling the atmosphere therein;    -   a discharge system 24 for recovering and expelling a portion of        the air being supplied into the minienvironment space 21; and    -   a prealigner 25 for performing coarse positioning of a substrate        (wafer) that has been placed in the minienvironment space 21 as        a sample.

The housing 22 comprises a top plate 221, a floor plate 222, and foursidewalls 223, constructed to isolate the minienvironment space 21 fromthe outside. To control the atmosphere in the minienvironment space, thegas circulation system 23, as shown in FIG. 3, comprises

-   -   a gas supply unit 231, installed within the minienvironment        space on the top plate 221, for cleansing a gas (air, in this        embodiment), and causing the cleansed gas to flow straight        downward in a laminar flow, through one or more gas discharge        outlets (not shown);    -   a recovery duct 232, placed within the minienvironment space on        the floor plate 222, for recovering air flowing downward toward        the floor plate; and    -   a conduit 233, connected between the recovery duct 232 and the        gas supply unit 231, for returning recovered air to the gas        supply unit 231.

In this embodiment, the gas supply unit 231 is configured so that about20% of the air supply is air that has been brought in from outside ofthe housing 22 and cleansed. The percentage of the supplied air that isbrought in from the outside, however, may be selected at the option ofthe user. The gas supply unit 231 is provided with either a HEPA (HighEfficiency Particulate Air) filter, or an ULPA (Ultra-Low PenetrationAir) filter, both of which are known filters, for producing the cleanair. The possibility remains, however, that some dust could beintroduced into the environment by the first transporter unit (to bedescribed later), which is located within the minienvironment space 21.To keep dust from the transporter from being deposited on a wafer, thedownward laminar flow (downflow) of clean air is supplied such that themain flow will pass over the wafer handling surfaces of the transporterunit. Therefore, the downflow discharge nozzles need not necessarily beplaced at the top plate, as shown in the drawing; it is only necessarythat they be located above the wafer handling surfaces of thetransporter unit. Neither is it necessary for the air to flow throughthe entire minienvironment space. In some cases, cleanliness may beensured by using an ‘ion wind’ for clean air. Sensors for monitoring aircleanliness may also be provided in the minienvironment space. Thisenables the system to be shut down promptly if air cleanliness is foundto have been degraded As shown in FIG. 1, an access port 225 is formedin that portion of the sidewall 223 of the housing 22 that is adjacentto the cassette holder 10. A shutter apparatus of known design can beprovided at the access port 225 to enable the access port to be closed.As for the flow rate of the laminar downflow created in the vicinity ofthe wafer, a flow rate of 0.3-0.4 meters per second, for example, wouldbe adequate. The gas supply unit need not necessarily be placed insidethe minienvironment space; it may be placed on the outside.

The discharge system 24 comprises

-   -   a suction duct 241 placed at the lower part of the transporter        unit, in a location below that of the aforementioned handling        surface of the transporter unit;    -   a blower 242 placed on the outside of the housing 22;

and

-   -   a conduit 243 connected between the suction duct 241 and the        blower 242.

In this discharge system 24, gas flowing down around the transporter,which could contain dust from the transporter, is attracted to thesuction duct 241 and discharged through the conduits 243 and 244, andthe blower 242, to the outside of the housing 22. This discharge gas maybe expelled into an external exhaust pipe (not shown) routed to thevicinity of the housing 22.

A prealigner 25 placed within the minienvironment space 21 is configuredto optically or mechanically detect either an orientation flat, or oneor more V-notches formed in the outer circumference of the wafer, and topre-align the position of thereof, in rotation about a wafer axis lineO-O, to an accuracy of approximately ±1 degree. The prealignerconstitutes a portion of the mechanism of the present invention, asrecited in the claims, for determining the coordinates of a sample, inwhich the function of the prealigner is to perform coarse alignment. Theconfiguration of the prealigner, per se, is known; therefore, itsconfiguration and operation will not be described.

A discharge system recovery duct (not shown) may also be provided nearthe bottom of the prealigner for expelling dust-bearing air from theprealigner to the outside.

Working Chamber

As shown in FIG. 1 and FIG. 2, a main housing 30, which forms theworking chamber 31, has a housing main unit 32. This housing main unit32 is supported by a housing support device 33, mounted on ananti-vibration device 37 (a vibration isolation device), which is placedon a pedestal frame 36. The housing support device 33 comprises arectangular frame 331. The housing main unit 32 is placed on, andfastened to, the frame 331, and comprises a floor plate 321 mounted onthe frame; a top plate 322; and sidewalls 323 that connect the top plate322 to the floor plate 321, and surround the space therebetween on foursides, for isolating the working chamber 31 from the outside. Althoughin this embodiment, the floor plate 321 is made of a relatively thicksteel plate to prevent its being distorted by the weight of equipmentsuch as the stage, other types of construction may also be used. In thisembodiment, the housing main unit and the housing support device 33 areassembled in a rigid structure. Vibrations from the floor on which thepedestal frame 36 is mounted are prevented from being transmitted to therigid structure by the vibration isolation device 37. An access port325, for passing wafers therethrough, is formed in the sidewall 323 ofthe housing main unit 32 that is adjacent to a loader housing (to bedescribed later).

The vibration isolation device may be an active, or a passive deviceusing air springs, magnetic bearings, etc. Since known constructions maybe used in both cases, the construction and functions of the devices,per se, will not be described. A vacuum atmosphere is maintained in theworking chamber 31 by a vacuum apparatus (not shown) that is also ofknown design. A control unit 2 for controlling the operation of theentire system is installed under the pedestal frame 36. This controlunit has a control system made up primarily of a main controller, acontrol controller, and a stage controller.

The main controller includes a man-machine interface through which anoperator performs various operations (executes instructions andcommands, inputs recipes, enters the ‘start inspection’ instruction,switches between automatic and manual inspection modes, inputs allnecessary commands when in manual inspection mode, etc.). In addition,the main controller communicates with the plant host computer; controlsthe vacuum evacuation system; controls the movement and positioning ofsamples (wafers, etc.); and sends other commands to, and accepts datafrom, the control controller and stage controller. Also included are astage vibration correction function that acquires image signals from theoptical microscope and feeds-back a stage motion variance signal to theelectron optical system for correction of image degradation; and anautomatic focus correction function that senses changes in the Zdirection (back and forth along the axis of a secondary optical system)of the sample observation position, and feeds-back this information tothe electron optical system, for automatic focus correction. Exchange offeedback signals, etc. going to the electron optical system and comingfrom the stage are performed through the control controller and stagecontroller, respectively. The control controller primarily takes care ofcontrolling the electron beam optical systems (by controllinghigh-precision power supplies for the electron gun, lenses, aligner, andExB separator, etc.).

The control performed by the stage controller is primarily related tostage motion. It is capable of controlling precise (μm-order) travel inthe X and Y directions of the stage within an error margin ofapproximately ±0.5 μm. With the present stage, rotation control (θcontrol) can be effected within an error margin of approximately ±0.3second of rotation.

Loader Housing

The loader housing 40 shown in FIGS. 1, 2, and 4, comprises a housingmain unit 43 that forms a first loading chamber 41 and a second loadingchamber 42. The housing main unit 43 comprises a floor plate 431, a topplate 432, four surrounding sidewalls 443, and a partition 433 forpartitioning the first loading chamber 41 from the second loadingchamber 42, constructed such as to isolate both loading chambers fromthe outside. An opening (access port 435) is formed in the partition 434for transferring wafers between the two loading chambers. Also, accessports 436 and 437 are formed in portions of the sidewalls 433 that areadjacent to the minienvironment unit and the main housing. The housingmain unit 43 of this loader housing 40 is supported by mounting it on aframe 331 of a housing support structure 33. Here too, then, theconfiguration is such that floor vibrations will not reach the loaderhousing 40.

The access port 436 of the loader housing 40 is aligned with the accessport 226 of the minienvironment housing 22, where a shutter 27 isprovided for selectively preventing communication between theminienvironment space 21 and the first loading chamber 41. The shutter27 comprises a seal 271 that surrounds the access ports 226 and 436 andis affixed in tight contact with the sidewall 433; a door 272 that worksin cooperation with the seal 271 to prevent passage of air through theaccess ports; and a driver unit 273 for moving the door.

Also, the access port 437 of the loader housing 40 is aligned with theaccess port 325 of the housing main unit 32, where a shutter 45 isprovided for selectively sealing off and preventing communicationbetween the second loading chamber 42 and the working chamber 31. Theshutter 45 comprises a seal 451 that surrounds the access ports 437 and325, and is affixed in tight contact with the sidewalls 433 and 323; adoor 452 that works in cooperation with the seal 451 to prevent passageof air through the access ports; and a driver unit 453 for moving thedoor. In addition, provided at the opening formed in the partition 434,is a shutter 46 with a door 461 that can be closed for selectivelysealing off and preventing communication between the first and secondloading chambers. These shutters are of known design, and theirconstruction and operation will therefore not be described.

Also, different methods are used for supporting the housing 22 of theminienvironment unit 20 and the loader housing. Therefore, to preventfloor vibrations from being transmitted through the minienvironment unitto the loader housing 40 and main housing 30, airtight anti-vibrationisolation damping device may be provided around the access port betweenthe housing 22 and the loader housing 40.

Provided in the first loading chamber 41 is a wafer rack 47 forsupporting a plurality of horizontal wafers (two, in this embodiment),with vertical separation therebetween. As shown in FIG. 5, the waferrack 47 has a rectangular baseplate 471 with support posts 472 attachedat the four corners thereof such as to stand erect and separate fromeach other. Formed in each of the support posts 472, at differentheights thereof, are two support portions 473 and 474, for placing andholding the periphery of a wafer W thereon such that the wafer can begrasped by an arm of the first and a second transporter unit (to bedescribed later).

The loading chambers 41 and 42 are constructed so that they can beatmosphere-controlled to maintain them in a high vacuum state (10⁻⁵-10⁻⁶Pa) by vacuum systems of known design comprising vacuum pumps (not shownin the drawing). Wafer contamination can be effectively prevented bymaintaining the first loading chamber 41 in a low vacuum state for useas a low vacuum chamber, and maintaining the second loading chamber 42in a high vacuum state for use as a high vacuum chamber. Thisconfiguration enables a wafer in the loading chamber, that is next inline to be inspected for defects, to be moved into the working chamberwithout delay. The use of a loading chamber configuration such theabove, in conjunction with multibeam principles of an electronic systemto be described later, improve the throughput of the defect detectingprocess; and in addition, enable the vacuum in the vicinity of theelectron source, where high vacuum storage conditions are required, tobe maintained in the highest possible vacuum state.

The first and second loading chambers 41 and 42 are connected to avacuum evacuation line, and to a vent line for performing purging withinert gas (e.g. dry pure nitrogen). (Neither line is shown in thedrawing.) This enables the loading chamber interiors to be brought toatmospheric pressure by purging the chambers using inert gas (i.e.,injecting inert gas into the chamber to prevent gases other than theinert gas, such as oxygen, from being adsorbed on surfaces). Apparatusof known configuration may be used for purging with inert gas, and theapparatus will therefore not be described in detail.

Moreover, in the inspection apparatus of the present invention, whichuses an electron beam, lanthanum hexaboride (LaB₆) is typically used asthe electron source for the electron optical system to be describedlater.

Once the LaB₆ has been heated to the high temperature state in whichthermal emission of electrons occurs, any oxygen coming in contact withthe electron source will greatly reduce its service life, and this musttherefore be avoided to the maximum extent possible. This can be assuredby using a controlled environment chamber such as described above at thestage from which wafers are transferred into the working chamber inwhich the electron optical system is installed.

The Stage

The stage 50 comprises

-   -   a fixed table 51 mounted on the floor plate 321 of the main        housing 30;    -   a Y table 52 that moves over the fixed table in a Y direction        (the direction perpendicular to the paper on which FIG. 1 is        printed);    -   an X table 53 that moves over the Y table in an X direction (the        horizontal direction of FIG. 1);    -   a turntable 54 that is rotatable on the X table; and    -   a holder 55 placed on the turntable 54.

A wafer is releasably held on a wafer seat 551 of the holder 55. Theholder may be of a known design for releasably securing a wafer eithermechanically, or by using an electrostatic chuck. The stage 50 isconfigured to effect the precise positioning, with respect to anelectron beam emitted by an electron optical system, of a wafer held onthe wafer seat 551 by the holder. This positioning is performed in the Xdirection, the Y direction, a Z direction (vertically in FIG. 1), and aθ direction (in rotation about an axis line perpendicular to the planeof the wafer seat), by using servo motors, encoders, and various sensors(not shown), to operate a plurality of tables such as those mentionedabove. The positioning in the Z direction may consist, for example, ofmaking fine adjustments in the position in the Z-direction of the waferseat on the holder. To do this, a position measurement device using avery-small-diameter laser (a laser interferometer ranging system usingthe principles of interferometry) can be used to sense a referenceposition of the wafer seat; and that position then controlled through afeedback circuit (not shown). Along with this, or instead of it, theposition of the notch or orientation flat of the wafer can be measured,and the horizontal and rotational position of the wafer with respect tothe electron beam controlled. To hold the amount of dust in the workingchamber to the absolute minimum, the stage servo motors 521 and 531, andthe encoders 522 and 532 are placed outside of the main housing 30.

It is also possible to standardize signals obtained by taking the X, Y,and rotation positions of the wafer with respect to the electron beam,and inputting them in advance to a signal detection circuit or imageprocessing circuit (to be described later). In addition, the wafer chuckmechanism provided on this holder is configured such that a waferchucking voltage can be applied to the electrodes of an electrostaticchuck to cause the wafer to be pressed against the holder at threepoints around the outer circumference of the wafer (preferably, atequally spaced points around the circumference), for positioning thewafer. The wafer chuck mechanism has two fixed positioning pins and onepush-pressure-type clamp pin. The clamp pin is configured for automaticchucking and automatic release, and includes contact points forapplication of voltage.

Overall Configuration of the Loader

The loader 60 comprises a robotic first transporter unit 61, which islocated in the housing 22 of the minienvironment unit 20; and a roboticsecond transporter unit 63, which is located in the second loadingchamber 42.

The first transporter unit 61 has a multi-jointed arm 612 that isrotatable about the axis (line O₁-O₁ of FIG. 1) of a driver unit 611.The specific configuration of the multi-jointed arm is up to the user,but the arm used in this embodiment has three sections configured to berotatable about each other. A first section of the arm 612 of the firsttransporter unit 61 (the section located nearest the driver unit 611) isinstalled on a shaft 613 that is rotated by a drive mechanism of knowndesign (not shown in the drawing) in the driver unit 611. In addition tobeing rotatable about the axis O₁-O₁ by the shaft 613, the entire arm612 can be extended and retracted (radially with respect to the axisO₁-O₁) through rotation of the individual sections relative to eachother. Provided on the outer end of the third section of the arm 612(the section most distant from the shaft 613 of the arm 612) is agrasping device 616 of known design (e.g., a mechanical or electrostaticchuck) for grasping and holding a wafer. The drive mechanism 611 isconfigured to be capable of being raised and lowered by an elevatormechanism 615 of known design.

This first transporter unit 61 extends its arm 612 in either a directionM1, or a direction M2, toward one of two cassettes c being held by thecassette holder, where its grasping device 616 grasps one of the wafersW contained in the cassette c (FIG. 12) and withdraws it. The arm thenretracts (to the state shown in FIG. 12), rotates to where it can extendtoward the prealigner 25 (direction M3), and stops. The arm then extendsonce more, and places the wafer W being held thereby on the prealigner25. After prealignment of the wafer, the above operation is reversed:the arm removes the wafer W from the prealigner 25, rotates to where itcan extend toward the second loading chamber 41 (direction M4), andstops. The arm then transfers the wafer to a wafer rack 47 in the secondloading chamber 41. When the wafer is grasped mechanically, it isgrasped near the periphery (a region extending 5 mm inward from theouter edge of wafer). The reason for this is that devices (circuitpatterns) are formed everywhere on the wafer except in this peripheralregion, and devices could be damaged, or defects created, by graspingthe wafer anywhere but in this peripheral region.

The second transporter unit 63 will not be described in detail becauseit is basically the same as the first transporter unit 61, differingonly in that it transports wafers between the wafer rack 47 and waferloading surface of the stage.

In the loader 60 just described, the first and second transporter units(61 and 63) transport wafers from the cassette holder holding thecassette, to the stage 50 in the working chamber 31, or they do the samething in reverse, with the wafers held substantially horizontalthroughout. The arms of the transporter units move up and down only towithdraw wafers from, or insert them in, a cassette; load wafers in, orremove them from, the wafer rack; or to place wafers on, or remove themfrom, the stage. This makes it possible to handle even very large wafers(wafers 30 cm in diameter, for example) smoothly.

Transfer of Wafers from a Cassette to the Working Chamber by the Loader

Described next, is the sequence for transferring a wafer from a cassettec being held in the cassette holder, to the stage 50, located in theworking chamber 31.

As mentioned earlier, a cassette holder 10 configured for manual loadingcan be used if the cassettes are to be set in it by hand, whereas ifcassettes are to be set in the loader automatically, a holder designedfor this type of operation may be used. In this embodiment, when acassette c is set in place on the elevator table 11 of the cassetteholder 10, the elevator table 11 is lowered by the elevator mechanism 12until the cassette c is aligned with the access port 225.

Once the cassette is lined up with the access port 225, a cover providedon the cassette opens. In addition, a tubular cover may also be providedbetween the cassette c and the minienvironment access port 225, forshielding the cassette interior and the minienvironment space from theoutside. If a shutter has been provided in the minienvironment unit 20for opening and closing the access port 225, that shutter is alsooperated to open the access port 225.

At this time, the arm 612 of the first transporter unit 61 will be inthe stopped state, facing in either the M1 or M2 direction (assume M1for this description). Now, when the access port 225 opens, the armcomes forward, and with its forward end, withdraws one of the waferscontained in the cassette. In this embodiment, alignment of the verticalpositions of the arm with the wafer to be removed from the cassette isperformed by the driver unit 611 of the first transporter unit 61, whichadjusts the vertical position of the arm 612. This could also beaccomplished, however, through vertical motion of the cassette holderelevator table, or by moving both the elevator table and the arm. Onceremoval of the wafer by the arm 612 is complete, the arm is retracted,and the shutter (if there is one) is operated to close the access port.Next, the arm 612 is rotated about the O₁-O₁ axis to a position whereinit can be extended in the M3 direction. The arm then extends, and placesthe wafer (either riding on the end of the arm or being grasped by achuck) on the prealigner 25. The prealigner properly positions therotational orientation of the wafer (its orientation in rotation about acenter axis perpendicular to the flat surface of the wafer) to within aprescribed number of degrees. When the positioning is complete and thewafer has been removed (on the end of the arm) from the prealigner 25,the first transporter unit 61 retracts the arm, and rotates to aposition wherein the arm is extended in the M4 direction. The door 272of the shutter 27 now operates to open the access ports 226 and 436,after which the arm 612 extends and places the wafer in the top or thebottom of the wafer rack 47, in the first loading chamber 41. Also,before the shutter 27 opens to transfer the wafer to the wafer rack 47,as described above, the opening 435 in the partition 434 is closed withan airtight seal by the door 461 of the shutter 46.

During the above wafer transport process, performed by the firsttransporter unit, a laminar flow (downflow) of clean air is suppliedfrom the gas supply unit 231 provided in the upper portion of theminienvironment unit housing, to keep dust from being deposited on thetop surface of the wafer during transport. A portion of the air aroundthe periphery of the transporter unit (in this embodiment, about 20% ofthe air supplied by the supply unit—mainly dirty air) is expelled to theoutside of the housing through the suction duct 241 of the dischargesystem 24. The remainder of the air is recovered and returned to the gassupply unit 231 through the recovery duct 232 provided at the floor ofthe housing.

When a wafer is placed in the wafer rack 47 in the first loading chamber41 of the loader housing 40 by the first transporter unit 61, theshutter 27 closes, sealing off the inside of the loading chamber 41.Then, after the first loading chamber 41 is purged of air and filledwith inert gas, the inert gas is also removed, creating a vacuumatmosphere in the loading chamber 41. For the vacuum in the firstloading chamber 41, a low level vacuum will do. Once a given level ofvacuum has been obtained in the first loading chamber 41, the shutter 46is operated to open the access port 434 (which had previously beensealed shut by the door 461). At this point the second transporter unit63 extends its arm 632, and with the grasping device at the end of thearm, takes one wafer from the wafer rack 47 (with the wafer eitherriding on the end of the arm or being grasped by a chuck). When thewafer has been completely withdrawn, the arm is retracted, and theshutter 46 again operates to seal off the access port 435 with the door461. Note that before the shutter 46 opened, the arm 632 had beenpre-positioned (facing in a direction N1) so that it could extend towardthe wafer rack 47. Also, in the above description, before the shutter 46was opened, the access ports 437 and 325 were closed by the door 452 ofthe shutter 45, to prevent communication between the second loadingchamber 42 and the working chamber 31; and then the second loadingchamber 42 was vacuum-evacuated.

After the shutter 46 closes the access port 435, the second loadingchamber is again evacuated, this time to a higher level of vacuum thanthat in the first loading chamber. While this is taking place, the armof the second transporter unit 61 is rotated to the position in which itcan be extended toward the stage 50 in the working chamber 31. At thestage, in the working chamber 31, the Y table 52 is moved in the upwarddirection of FIG. 2, until it reaches a position in which the centeraxis X_(o)-X_(o) of the Y table 53 is substantially aligned with the Xaxis line X₁-X₁ passing through the axis of rotation O₂-O₂ Of the secondtransporter unit 63. The X table 53 is moved as far as possible to theleft in FIG. 2. The stage waits in this state.

When the second loading chamber reaches about the same level of vacuumas the working chamber, the door 452 of the shutter 45 operates to openthe access ports 437 and 325. The arm then extends, causing the end ofthe arm, which is holding the wafer, to approach the stage in theworking chamber 31, and it places the wafer on the wafer seat 551 of thestage 50. When this placement of the wafer is completed, the armretracts, and the shutter 45 closes the access ports 437 and 325.

The above describes system operation in terms of transporting a waferfrom a cassette c to the stage. Once the wafer has been loaded on thestage, and its processing completed, it is transported from the stageand returned to a cassette c in an operation that is the reverse of thatdescribed above. Also, because a plurality of wafers can be loaded inthe wafer rack 47, while one wafer is being transferred between thewafer rack and the stage by the second transporter unit, another wafercan be transferred between the cassette and the wafer rack by the firsttransporter unit. This provides a more efficient inspection process.

Examples of Modified Versions of the Working Chamber

One example of a modification in terms of the way the main housing issupported is shown in FIG. 6. In the modified version shown in FIG.6(A), a housing support device 33 a is made of a thick rigid rectangularplate 331, on which a housing main unit 32 a is mounted. Accordingly,the floor plate 321 a of the housing main unit 32 a is of thinnerconstruction than the floor plate in the embodiment described above. Inthe modified version shown in FIG. 6(B), the housing main unit 32 b andloader housing 40 b are supported by suspending them from by the frame336 b of a housing support device 33 b. A plurality of vertical frames337 b (fastened to the frame 336 b) are fastened to the housing mainunit 32 b, with the bottoms of these vertical frames fastened to thefour corners of a floor plate 321 b of the housing main unit 32 b, suchthat the sidewalls and top plate are supported by the floor plate.Placed between the frame 336 b and a pedestal frame 36 b is an vibrationisolation device 37 b. The loader housing 40 is also suspended by ahanger 49 b fastened to the frame 336. Because the modified housing mainunit 32 b of FIG. 6(B) is supported by suspension, the main housing andthe various equipment in contains (as a whole) has a lower center ofgravity. The method of supporting the main housing and loader housingincluded in the above modification example is designed to preventvibrations from being transmitted from the factory floor to the mainhousing or loader housing.

In another example of a modified version not shown in the drawings, onlythe housing main unit of the main housing is supported from below by ahousing support device, the loader housing being supported above thefloor by the same method as that used to support the adjacentminienvironment unit.

In yet another example of a modified version not shown in the drawings,only the housing main unit of the main housing is supported bysuspending it from a frame, the loader housing being supported above thefloor by the same method as that used to support the adjacentminienvironment unit.

Electron Optical System

An electron optical apparatus 70 comprises a column 71 installed in thehousing main unit 32. Provided in the column 71, as shown schematicallyin FIGS. 7 and 8, are a primary electron optical system (hereinafter,‘primary optical system’) 72, a secondary electron optical system(hereinafter, ‘secondary optical system’) 74, and a detector 76. Theprimary optical system 72 is the optical system that irradiates thesurface of a wafer W (the sample) with electron beams. This primaryoptical system 72 comprises

-   -   an electron gun 721, for emitting an electron beam;    -   an electrostatic condenser lens 722, for converging the primary        electron beam emitted by the electron gun 721;    -   a multi-aperture plate 723 placed below the condenser lens 722        and having multiple apertures formed therein, for forming the        primary electron beam into multiple primary electron beamlets;    -   an electrostatic demagnification lens 724, for demagnifying the        primary electron beams;    -   a Wien filter (ExB separator) 725; and    -   an objective lens 726;

all of which, as shown in FIG. 7, are arranged in the above listedsequence with the electron gun 721 at the top, such that the opticalaxis of the primary electron beam emitted by the electron gun isperpendicular to the surface of a sample S (e.g., a wafer W).

The electron gun uses a thermionic electron beam source.

The emitter material is LaB₆, but another material may be used as longas it has a high melting point (a low vapor pressure at hightemperatures) and low work function. There are two ways to obtainmultiple beams: one is to draw one electron beam from one emitter (anemitter with one tip), and the other is to form multiple tips on anemitter and draw multiple beams from them. In this embodiment of thepresent invention, primarily the latter of these is used. An example ofanother type of electron beam source that could also be used is thethermal field emission electron beam. In a thermionic electron beamsource, emission of electrons is obtained by heating the emittermaterial. In a thermal field emission electron beam source, a strongelectric field is applied to the emitter material to obtain electronemission, and the electron beam emitter is also heated, to stabilizeemission.

An ExB separator, as shown in FIG. 14, is made up of an electrostaticand an electromagnetic deflectors. The electrostatic deflector comprisesa pair of poles (electrostatic deflection poles) 725-1 in a vacuumchamber, for generating an electric field in the x-axis direction. Theseelectrostatic deflection poles 725-1 are mounted on a vacuum bulkhead725-3, with insulating spacers 725-2 therebetween. The distance D (thedistance between these poles) is set to be less than 2 L (the length ofthe electrostatic deflection poles 725-1 in the y-axis direction).Establishing this relationship will provide a fairly large range overwhich the strength of the electric field about the z-axis (optical axis)is uniform. Ideally, however, a larger range of uniform electrical fieldstrength will be obtained by setting D<L.

That is, since the electric field strength cannot be not uniform over arange extending from the ends of the poles to D/2, the region over whichit will be substantially uniform is a central region extending to 2 L-D,excluding the regions nearest the ends, where the field strength will benon-uniform. Thus a relationship 2 L>D must be established for a regionof uniform electric field strength to even exist, and a larger regioncan be obtained by setting L>D.

An electromagnetic deflector is provided outside of the vacuum bulkhead725-3 for generating a magnetic field in the y-axis direction. Thiselectromagnetic deflector comprises electromagnetic coils 725-4 and725-5, for generating magnetic fields in the x- and y-axis directions,respectively. The magnetic field in the y-axis direction could becreated by the coil 725-5 only, but the coil 725-4 is added to improvethe orthogonality between the electric and magnetic fields. That is, theorthogonality between electric and magnetic fields is improved becausethe component of the magnetic field generating by the coil 725-4 in the−x axis direction cancels the component of the magnetic field created bythe coil 725-5 in the +x axis direction.

FIG. 15 shows another embodiment of the ExB separator of the presentinvention. This embodiment differs from the one shown in FIG. 14 in thatit has six electrostatic deflection poles 725-1. Applied to each ofthese electrostatic deflection poles 725-1 is a voltage proportional toK·cos θ_(i) (i=0, 1, 2, 3, 4, 5), where k is a constant, and θ_(i) isthe angle formed between a line from the center of the pole piece to theoptical axis (z axis) and the direction of the electric field (x-axisdirection in this case). The value of the angle θ_(i) is arbitrary.

In the embodiment shown in FIG. 15, as well, coils 725-4 and 725-5 areprovided for generating magnetic fields in the x-axis and y-axisdirections, and for improving orthogonality, the same as in the firstembodiment. This embodiment, however, provides a larger region ofuniform electric field strength than can be obtained in the embodimentof FIG. 14.

In the ExB separator shown in FIG. 14 and FIG. 15, the coils forgenerating magnetic fields are formed as saddle-type coils. Toroidalcoils, however, could also be used.

To eliminate the effects of field curvature aberrations in thedemagnification lens 724 and the objective lens 726, a plurality ofapertures 723 a (nine, in this embodiment) formed in the multi-apertureplate 723, are arranged in a circle having the optical axis at itscenter, spaced so as to lie at equal intervals Lx in the X direction ofthe projected image, as shown in FIG. 8.

The secondary optical system 74 comprises magnification lenses 741 and742 (a two-stage electrostatic lens) through which secondary electronsseparated from the primary optical system by the ExB separator 725 arepassed; and a detector multi-aperture plate 743. The apertures 743 aformed in the detector multi-aperture plate 743 correspond one-for-onewith the apertures 723 a formed in multi-aperture plate 723 of theprimary optical system.

The detector 76 comprises a plurality of detector elements 761 (nine, inthis embodiment), each of which is placed in close proximity to thecorresponding apertures 743 a of the multi-aperture plate 743 of thesecondary optical system 74; and an image processor 763 that iselectrically connected through A/D converters 762 to the detectorelements 761 of the detector 76.

Electron Optical System Operation

Next, the operation of an electron optical apparatus 70 configured asdescribed above will be described.

The primary electron beam emitted from the electron gun 721 is convergedby the condenser lens 722 of the primary optical system 72 to form acrossover at a point P1. This primary electron beam that is converged bythe condenser lens 722 is formed by the plurality of apertures 723 a ofthe multi-aperture plate into a plurality of primary electron beamlets,which are then demagnified by the demagnification lens 724 to beprojected at a position P2. After first being focused at the positionP2, the beamlets are again focused by the objective lens 726 on thesurface of a wafer W.

The deflector 727 placed between the demagnification lens 724 and theobjective lens 726 deflects the primary electron beamlets for scanningthem over the surface of the wafer W. FIG. 16 shows an example of oneway in which the surface of the wafer W may be scanned by the primaryelectron beam. In this example four equally spaced primary electronbeamlets 101-104 scan from left to right in the drawing. When thebeamlets reach the right ends of their scans, the stage on which thewafer is being supported moves upward (in the drawing) by a prescribedstep distance, as the beamlets return to the left end of the scan. Thisprocess then repeats until the required region of the wafer has beenscanned.

Thus the sample S (wafer W) is irradiated at a plurality of points bythe plurality of primary electron beamlets (nine, in this embodiment),causing secondary electrons to be emitted from these points. Thesesecondary electrons are attracted to, and tightly converged by, theelectric field of the objective lens 726; and then are deflected by theExB separator 725 to be injected into the secondary optical system 74.The image formed by the secondary electrons comes into focus at aposition P3, which is nearer the deflector 725 than is the position P2.Although the energy of the primary electron beam at the wafer is 500 eV,the energy of secondary electrons is only a few eV.

Each of the secondary electron images focused at the position P3 ismagnified by the two magnification lenses 741 and 742, is again focusedat the corresponding aperture 743 a of the multi-aperture detector plate743, and passes through the aperture to be detected by the detectorelement 761 placed at its corresponding aperture 743 a. The detectorelement 761 converts the detected electron beamlet to an electricalsignal representative of the intensity of the secondary beamlet.Electrical signals thus converted are output from each of the detectorelements 761, after which they are converted to a digital signal by anA/D converter 762, and input to the image processor 763. The imageprocessor 763 converts its digital signal inputs to image data. The scansignal used to deflect the primary electron beam is also supplied to theimage processor 763 to be used, along with its digital input, to form animage of the surface of the wafer. In a comparator (not shown), thisimage is compared to a standard pattern input to a settings unit (notshown) beforehand, thus to detect (evaluate) pattern defects in theinspected wafer.

In addition, the line width of a pattern formed on a wafer W can bemeasured by performing a registration operation to move the pattern tobe measured near the optical axis of the primary optical system,performing a line scan to obtain a line width signal, and properlycalibrating the signal.

Also, with respect to the image, it should be noted that although theabove description covered the case in which the obtaining of onlysecondary electron images was selected, images can be obtained not onlyfor secondary electrons, but also for scattered electrons, andback-scattered electrons as well.

Also, in a system such as described above, in which primary electronbeamlets passed through the apertures of the multi-aperture plate 723 ofthe primary optical system are brought to a focus at the surface of thewafer W, and secondary electrons emitted from the wafer are imaged onthe detector element 761, special emphasis must be placed on minimizingthe effects of the following three types of aberration occurring in theprimary optical system: coma, axial chromatic aberration, and fieldastigmatism.

Also, the separation between the primary beamlets is related to thesecondary optical system in that crosstalk between beamlets can beeliminated by separating the primary beamlets from each other by adistance greater than the aberration of the secondary optical system.

Also, during operation of the electron optical system, evaporation ofsample material, etc. takes place. Over time, this results in theaccumulation of contamination material on various optical elements suchas deflectors, the effects of which can degrade system operation. Thisaccumulated contamination must therefore be removed on a periodic basis.This can be accomplished by using electrodes in the vicinity of areashaving accumulations of contamination material, in a vacuum, to excite aplasma of hydrogen, oxygen or fluorine, thus to oxidize and remove onlythe contamination material.

Electron Optical System for Precharge Unit

As shown in FIG. 1, a precharge unit 81 is installed in the workingchamber 31, near to the column 71 of the electron optical apparatus 70.The present inspection apparatus is a type of system in which inspectionof a device pattern, etc., formed on the surface of a substrate, i.e., awafer (the sample) is performed by scanning the wafer with electron beamirradiation, and using data from the secondary electrons produced bythis electron beam irradiation to obtain information about the wafersurface.

During this process, depending on factors such as the material of thewafer and the energy of the irradiating electrons, ‘charge-up’(accumulation of electrical charge on the surface of the wafer) canoccur. In addition, the charge can be strong in some parts of thesurface, and weak in others. When such charge variations exist over thesurface, they produce variations in the secondary electron signal, andaccurate information can no longer be obtained. Therefore, in thepresent embodiment, a precharge unit 81, comprising a charged particleirradiator 811, is provided, to prevent such charge variations. In orderto avoid charge variations, before the prescribed location to beinspected on a wafer is irradiated with electrons, it is firstirradiated with charged particles from the charged particle irradiator811. The wafer is then checked for charge-up beforehand by forming animage of the wafer surface and evaluating that image, and the operationof the precharge unit 81 is then based on this evaluation.

Such charge variations can also be removed by blurring the primaryelectron beam in the precharge unit.

Precharging can also be used to inspect for electrical defect in a waferinspection sample. Assume, for example, that a location originallyintended to be electrically insulating has for some reason becomeconductive. In a comparison, the charge in the defective location willappear different from that in a location that is a properly insulatedportion. Therefore, when the wafer is irradiated with the primaryelectron beam and inspected based on secondary electrons, it will bepossible to check for electrical defects on the wafer.

Electrical Potential Application System for the Sample

The yield at which secondary electrons are emitted from a wafer isinfluenced by the electrical potential of the wafer. Based on this fact,the electrical potential application system for sample 83 (FIG. 9)controls the emission of secondary electrons by applying a potential of± a few volts to the mount on which the stage carrying the wafer isinstalled. This electrical potential application system also serves toreduce the landing energy possessed by the irradiating electrons,putting the energy of the irradiating electrons at the wafer at roughly100-500 eV.

As shown in FIG. 9, the electrical potential application system 83comprises a voltage application device 831 electrically connected to amounting surface 541 of the stage 50, and a charge-up investigation andvoltage determination system (hereinafter, ‘investigation anddetermination system’) 832. The investigation and determination system832 comprises a computer monitor 833 electrically connected to the imageprocessor 763 of the detector 76 of the electron optical apparatus 70;an operator 834 for monitoring the monitor 833; and a CPU 835 that iscontrolled by the operator 834. The CPU 835 supplies signals to theabove voltage application device 831 and to the deflector 727.

The above electrical potential application system is designed to find apotential at which it would not be likely for the sample (wafer) tocharge up, and to apply that potential.

Calibration Mechanism for the Electron Optical System

As shown in FIG. 10, a calibration mechanism 85 comprises a plurality ofFaraday cups 851 and a plurality of Faraday cups 852 placed at aplurality of locations at the side of the wafer-mounting surface 541, onthe aforementioned turntable, for measurement of beam current. TheFaraday cups 851 are for fine beams (approximately φ2 μm), and theFaraday cups 852 are for broad beams (approximately φ30 μm). Thefine-beam Faraday cups 851 are used to measure beam profiles whilerotating the turntable in steps. Total beam current is measured by thebroad-beam Faraday cups 852. The Faraday cups 851 and 852 are placedsuch that their top surfaces are at the same level (height) as the topsurface of a wafer W placed on the wafer-mounting surface 541. In thisconfiguration, the primary electron beam emitted by the electron gun isalways monitored. The reason for doing this is that the electron guncannot always emit a constant beam, and variations in the amount ofemission can therefore occur during use.

Alignment Control System

The alignment control system 87 is a system for positioning a wafer Wwith respect to the electron optical axis using the stage 50. It isdesigned to control operations such as rough alignment of the wafer inwide field observation using the optical microscope 871 (a less accuratemeasurement than one by the electron optical system); high magnificationalignment using the electron optical systems of the electron opticalapparatus 70; focus adjustment; setting of the inspection area; andpattern alignment. The reason for using an optical microscope to inspectthe wafer at low magnification when performing wafer alignment is thatit provides a convenient way of detecting the electron beam alignmentmarks for automatic inspection of the wafer pattern with the electronbeam in narrow field observations of the pattern.

The optical microscope 871 is provided in the housing (it may beprovided such as to be movable within the housing). A light source (notshown) for operating the microscope is also provided within the housing.The alignment control system 87 is shown schematically in FIG. 11. Toobserve the point to be observed (on the wafer), at low magnification,that point is moved into the field of the optical microscope by movingthe x-stage 53 of the stage 50 in the X direction. A wide-fieldobservation of the wafer is then performed using the optical microscope871. The location to be observed on the wafer is found and displayed ona computer monitor 873 through a CCD 872, and the observation positionis roughly determined. During this procedure, the magnification of themicroscope may be changed between low and high magnification.

Next, the stage 50 is moved a distance corresponding to δx, theseparation between the optical axis of the electronic optical apparatus70 and the optical axis of the microscope 871, to thus move theobservation point (on the wafer), determined in advance using themicroscope, into the field of the electron optical apparatus. When sodoing, since δx, the distance between the axis O₃-O₃ (the electronoptical axis) and the axis O₄-O₄ (the optical axis of the microscope871) is known in advance, the observation point can be moved to theobserved position by moving by an amount equal to the value of δx. Also,although in this example, it was assumed that the positions of both axesare displaced only in the X axis direction, they may actually bedisplaced in both the X and Y axis directions. After the point to beobserved has been moved to the observed position of the electron opticalapparatus, an SEM photograph of the observed point is taken at highmagnification by the electron optical system. The SEM photograph is thenstored in memory and displayed on the monitor 765.

After the point on the wafer to be observed has been displayed at highmagnification on the monitor by the electron optical system, thedisplacement of the position of the wafer in rotation about the centerof rotation of the turntable 54 of the stage 50 (i.e. δθ, thedisplacement of the wafer in rotation about the electron optical systemoptical axis O₃-O₃), is detected by a known method. The displacement inthe X and Y directions of the position of a prescribed pattern withrespect to the electron optical system is also detected. Then, waferalignment is performed by controlling movement of the wafer stage 50based on these detected values, along with separately obtained data onalignment marks provided on the wafer, and data on the wafer patternshape.

Inspection Sequence (Summary)

FIG. 13 summarizes the procedure for inspecting wafers (samples) usingthe electron optical apparatus. First, using the optical microscope, thepositions of dies are checked, and heights of locations are detected andstored, as required. Next, recipe data is entered into the system forthe wafer type being inspected (information on the last processcompleted, whether the wafer size is 20 cm or 30 cm, etc.), thelocations to be inspected are specified, and optical system settings andinspection conditions are entered. Normally, defect inspection isperformed in real time while acquiring images. Cell-to-cell comparisons,die comparisons, etc. are performed by a high speed data processingsystem provided with appropriate algorithms. Inspection results areoutput to a CRT display and/or stored in memory. Defects can also beautomatically classified in real time as to type, i.e., particledefects, shape abnormalities (pattern defects), or electrical defects,which can be further classified in terms of defect size, and ‘killerdefects’ (serious defects that will prevent the chip from being used).Electrical defect detection can be accomplished through detection ofelectrical potential contrast abnormalities. For example, when alocation having poor continuity is irradiated with an electron beam (ofapproximately 500 eV) it will normally take on a positive charge,increasing its contrast, and thus making it distinguishable from goodlocations. The electron beam irradiation system used for this purposemay be a separate system, other than the one used for inspection. Such aseparate system would be a low electrical potential energy electron beamirradiation system for enhancing the contrast due to difference inpotential. Before irradiating the sample region with the inspectionelectron beam, it is irradiated with the low electrical potential energyelectron beam. If a projection lithography technique capable ofproducing the required positive charge is used to irradiate the samplewith the inspection electron beam itself, however, depending on theapplication, a special system for generating a low electrical potentialelectron beam may not be required.

Defects can also be detected from the differences in contrast producedwhen a potential that is negative or positive with respect to areference potential is applied to a sample such as a wafer. Thedifferences in contrast in this case are caused by differences in theease with which currents flow through device elements in the forward andreverse directions. This technique can also be used with line width andalignment accuracy measurement systems.

Effects of the Inspection Apparatus in the Above Embodiment

In particular, the following effects can be brought to fruition throughapplication of the inspection apparatus of the above embodiment:

-   a. High throughput processing of inspection samples can be achieved    through effective functional combination of the various equipment    components that make up the multi-beam inspection apparatus.-   b. While samples are being inspected, monitoring for the presence of    dust in controlled environment space can be conducted via sensors    provided within the environment.-   c. A precharge unit provided in the apparatus substantially reduces    the influence of electrical charge on inspections, even on insulated    wafers.    Maintenance of Vacuum in the Working Chamber (Stage Improvement 1)

When ultra-high-precision positioning of the stage that supports thewafer (the inspection sample in the apparatus of the present invention)is required, the structure for supporting the stage uses static pressurebearings that support the stage without touching it. During operation,high pressure gas is expelled from the bearings. To keep this highpressure gas from entering directly into the vacuum chamber, adifferential pumping mechanism for expelling the gas is formed adjacentto the bearing, to maintain the proper vacuum in the vacuum chamber.

An example of a prior stage for such a system is shown in FIGS. 17(A)and (B). In the configuration shown in the drawing, an electron beamirradiator 2-2 (the output end of a column 1-2) of an electron beaminspection apparatus that generates an electron beam for irradiating asample is mounted on a housing 14′-2 that forms a vacuum chamber C. Toform a vacuum in the column, the column is evacuated through a vacuumline 18-2. The vacuum chamber C is evacuated through a vacuum line19′-2. The electron beam irradiates a sample (wafer) by passing throughthe output end 2-2 of the column 1-2, under which the sample is placed.

The wafer W is releasably retained on a sample table t by a knownmethod, and the sample table t is installed on the top surface of aY-movable section 4′-2 of an XY stage (hereinafter ‘stage’) 3′-2.Provided on this Y-movable section 4′-2, facing a guide surface 5 a′-2(comprising the left and right sides and the bottom in FIG. 17(A)) of anX-movable section 5′-2 of the stage 3-2, are a plurality of staticpressure bearings 9′-2, for moving the stage in the Y direction (leftand right in FIG. 17(B)) through the action of the static pressurebearings 9′-2, as an extremely narrow gap is maintained between themovable section and the guide surface. A differential gas dischargemechanism is also provided around the static pressure bearings, toprevent high pressure gas being supplied to the static pressure bearingsfrom leaking into the vacuum chamber C. This mechanism is shown in FIG.18. Double grooves (the grooves g1-2 and g2-2) are formed around thestatic pressure bearings 9-2. These grooves are constantly beingevacuated by a vacuum pump, via a vacuum line (not shown). Thisconfiguration enables the Y-movable section 4′-2 to be non-contactinglysupported in a vacuum such as to be movable in the Y direction. Thesedouble grooves g1-2 and g2-2 are formed in the surface in which thestatic pressure bearing 9′-2 of the movable section 4′-2 is provided,such as to surround the static pressure bearings.

As is evident from FIGS. 17(A) and (B), the shape of the X-movablesection 5′-2, in which this Y-movable section 4′-2 is provided, is thatof an open top box. Exactly the same kind of static pressure bearingsand grooves are also provided in this X-movable section 5′-2, thusenabling it to be non-contactingly supported in a stage mount 6′-2 so asto be movable in the X direction.

By combining the movements of the Y-movable section 4′-2 and X-movablesection 5′-2, the sample W can be moved to any location (horizontally)relative to the output end of the column (electron beam irradiator) 2-2,for irradiating the desired location on the sample with the electronbeam.

A problem with the above system, in which static pressure bearings arecombined with a differential pumping mechanism, however, is that thedifferential pumping mechanism makes the stage larger and more complexthan static pressure bearing-type stages used at atmosphere pressure.The stages are also more expensive, and less reliable.

Accordingly, the present invention eliminates the differential dischargemechanism of the XY stage to provide a simple configuration while stillproviding an electron beam inspection apparatus in which the vacuum inthe working chamber is maintained at the required level.

Electron Beam Inspection Apparatus With an Improved Stage

Following is a description of an embodiment of the electron beaminspection apparatus of the present invention with an improved stage.Items in the drawing of the improved stage that are the same as items ofthe prior stage shown in FIG. 17(A) and FIG. 17(B) will retain the samereference numbers in this description. Also, for the purposes of thisspecification, the term ‘vacuum’ shall be understood to mean vacuum asit is commonly understood in the art.

A first embodiment of the electron beam inspection apparatus with animproved stage is shown in FIG. 19. An electron beam irradiator 2-2 (theoutput end of a column 1-2) that emits an electron beam toward a sampleis installed in a housing 14-2 that forms a vacuum chamber C. An XYstage 3-2 is configured such that a sample W placed on a table that ismovable in the X-direction (the left-right direction of FIG. 19) can beplaced directly under the column 1-2. The sample W can then bepositioned by the high-precision XY stage 3-2 such as to cause theelectron beam to accurately irradiate any desired location on thesurface of the sample.

A mounting plate 6-2 of the XY stage 3-2 is attached to the floor plateof a housing 14-2, and a Y table 5-2 that is movable in the Y direction(the direction perpendicular to the page of FIG. 19) is placed on themounting plate 6-2. A pair of Y-direction guides 7 a-2 and 7 b-2 havingchannels formed in one side thereof, are placed on the mounting plate6-2 such that their channels face the Y table. Also, formed on the leftand right side surfaces of the Y table 5-2 (left and right as shown inFIG. 19), are protrusions that protrude into the channels of theY-direction guides 7 b-2 and 7 a-2. The channels extend nearly theentire length of the Y-direction guides. Provided on the top, bottom,and side surfaces of the protrusions that protrude into the channels,are static pressure bearings 11 a-2, 9 a-2, 11 b-2, and 9 b-2, of knownconstruction. Through this configuration, the Y table 5-2 isnon-contactingly supported in the Y-direction guides 7 a-2 and 7 b-2 byhigh pressure gas blown through these static pressure bearings, suchthat it can move smoothly back and forth in the Y direction. A linearmotor 12-2 of known construction is placed between the mounting plate6-2 and the Y table 5-2 for driving the Y-table in the Y direction. Highpressure gas is supplied to the Y table by a flexible high pressure gassupply line 22-2, and high pressure gas is supplied through a gas duct(not shown) formed within the Y table to the static pressure bearings 9a-2 through 11 a-2 and 9 b-2 through 11 b-2. High pressure gas suppliedto the static pressure bearings is blown into gaps (of a few microns toa few tens of microns) formed between guide surfaces opposite theY-direction guides, to provide accurate positioning of the Y table withrespect to the guide surfaces, in the Z direction (up and down in FIG.19) and X direction.

Placed on the Y table, such as to be movable in the X direction (leftand right in FIG. 19), is an X table 4-2. A pair of X-direction guides 8a-2 and 8 b-2 (only 8 a-2 is shown), are provided on the Y table 5-2,with the X table 4-2 therebetween. These X-direction guides are of thesame construction as the Y-direction guides 7 a-2 and 7 b-2 for the Ytable. In these X-direction guides as well, channels are formed on thesides of the guides that face the X table. Also, protrusions thatprotrude into the channels are formed on the sides of the X table thatface the X-direction guides. The channels extend nearly the entirelength of the X-direction guides. Provided on the top, bottom, and sidesurfaces of the protrusions on the X-table 4-2 that protrude into thechannels, are the same kind of static pressure bearings 11 a-2, 9 a-2,10 a-2, 11 b-2, 9 b-2, and 10 b-2 (not shown). A linear motor 13-2 ofknown construction is placed between the X table 4-2 and the Y table 5-2for driving the X-table in the X direction. High pressure gas issupplied to the X table 4-2 by a flexible high pressure gas supply line21-2, for supplying high pressure gas to the static pressure bearings.This high pressure gas is blown from the static pressure bearings towardthe guide surfaces of the X-direction guides for non-contactinglysupporting the X table 4-2 and accurately positioning it with respect tothe Y-direction guides. The vacuum chamber C is evacuated by vacuumpumps of known construction, connected to the vacuum lines 19-2, 20 a-2,and 20 b-2. The vacuum lines 20 a-2 and 20 b-2 are passed throughmounting plate 6-2 so that their input ends (the ends inside the vacuumchamber) open at the surface of the mounting plate near locations wherehigh pressure gas is expelled from the XY stage 3-2. This almostcompletely prevents pressure fluctuations from developing within thevacuum chamber due to the high pressure gas being blown out of thestatic pressure bearings.

A differential discharge mechanism 25-2 is provided around (encircling)the electron beam irradiator 2-2 (output end) of the column 1-2. Thisdischarge mechanism ensures that the pressure within an electron beamirradiation space 30-2 will remain sufficiently low, even if thepressure within the vacuum chamber C is high. An annular member 26-2 ofthis differential discharge mechanism 25-2 is installed around(encircling) the electron beam irradiator 2-2. This annular member ispositioned relative to the housing 14-2 such as to form an ultra-finegap 40-2 (a gap of from a few microns to a few tens of microns) betweenthe bottom surface of the annular member (the surface at the sample Wend) and the sample. An annular channel 27-2 formed in the bottomsurface of the annular member is connected to a vacuum pump (not shown)through a exhaust tube 28-2. High-pressure gas from the ultra-fine gap40-2 is expelled through the annular channel 27-2 and the dischargeoutlet 28-2, as will be any gas molecules from the vacuum chamber C thatpenetrate into the space 30-2 surrounded by the annular member 26-2.This arrangement makes it possible to maintain the required low pressurewithin the electron beam irradiation space 30-2 so that electron beamirradiation can be performed without problems. Double or triple annularchannels may be provided, depending on pressures within the chamber Cand the electron beam irradiation space 30-2.

Dry nitrogen is commonly used as the high pressure gas supplied to thestatic pressure bearings. If possible, however, it is preferable to usean inert gas of higher purity. If gas containing traces of moisture oroil is used, molecules of these impurities can adhere to the insidesurfaces of the housing that forms the vacuum chamber, the surfaces ofcomponents that make up the stage, and the surfaces of samples, and thelevel of vacuum within the electron beam irradiation space will bedegraded.

In apparatus such as that described above, samples W are not normallyplaced directly on the X table, but rather on a sample holder that iscapable of releasably holding the sample and making fine adjustments inthe positioning thereof with respect to the XY stage 3-2. To simplifythe description, however, since neither the use of such a sample holdernor the construction thereof is related to the gist of the invention ofthe present application, its description has been omitted.

Since static pressure bearing-type stage mechanisms designed for use atatmospheric pressure can be used substantially as-is in theabove-described electron beam inspection apparatus, it is possible torealize a high precision XY stage for an electron beam inspectionapparatus that is the same size, and is otherwise equivalent to, thehigh precision atmospheric pressure stages used in lithography systems,etc., and at about the same cost.

Moreover, the configuration and placement of the static pressure guidesand actuator (linear motor) described above constitute only one possibleembodiment; any static pressure guide or actuator that can be used inthe atmosphere may be used here.

Next, a quantitative example of the size of the annular member 26-2 ofthe differential discharge mechanism, and the annular channel formedtherein will be discussed with reference to FIG. 20. This example hasdouble annular channels 27 a-2 and 27 b-2, separated from each other inthe radial direction.

The flow rate of the high pressure gas to the static pressure bearingsis normally around 20 L/min (converted to atmospheric pressure terms).If a vacuum chamber C is evacuated by a dry pump having a pumping speedof 20,000 L/min, through a cylindrical vacuum line 2 meters long and 50mm in inside diameter, the pressure in the vacuum chamber will be about160 Pa (approximately 1.2 Torr).

Under these conditions, if the dimensions of the differential pumpingmechanism annular member 26-2 and its annular channel, etc. are as shownin FIG. 20, then a pressure of 10⁻⁴ Pa (10⁻⁶ Torr) can be obtainedwithin the electron beam irradiation space 30-2.

A second embodiment is shown in FIG. 21. The vacuum chamber C formed bythe housing 14-2 is connected to a dry vacuum pump 53-2 through vacuumlines 74-2 and 75-2. The annular channel 27-2 of the differentialpumping mechanism 25-2 is connected, through a vacuum line 70-2connected to the pumping outlet 28-2, to a turbomolecular pump 51-2 (anultra-high vacuum pump). In addition, the interior of the column 1-2 isconnected to a turbomolecular pump 52-2 through a vacuum line 71-2connected to the its discharge outlet 18-2. These turbomolecular pumps51-2 and 52-2 are connected through vacuum lines 72-2 and 73-2 to a dryvacuum pump 53-2.

In this drawing, a single dry vacuum pump is assigned double duty as theroughing pump for the turbomolecular pumps and also as the vacuumevacuation pump for the vacuum chamber. However, depending on factorssuch as the flow rate of the high pressure gas supplied to the XY stagestatic pressure bearings, the internal surface area and volume of thevacuum chamber, and the inside diameter and length of the vacuum lines,there may be cases in which a separate dry vacuum pump system would beappropriate.

High-purity inert gas (N₂, Ar, etc.) is supplied to the static pressurebearings of the XY stage 3-2 through the flexible lines 21-2 and 22-2.Molecules of these gases blown from the static pressure bearings aredispersed in the vacuum chamber and pumped through pumping outlets 19-2,20 a-2, and 20 b-2, by the dry vacuum pump 53-2. Any of these moleculesthat manage to penetrate into the differential discharge mechanism orelectron beam irradiation space are drawn out of the annular channel27-2 or the output end of the column 1-2, to be discharged through thedischarge outlets 28-2 and 18-2 by the turbomolecular pumps 51-2 and52-2. After pumping from the turbomolecular pumps, the molecules areagain pumped by the dry vacuum pump 53-2. In this manner, high-purityinert gas supplied to the static pressure bearings is collected andpumped by the dry vacuum pump.

The pumping outlet of the dry vacuum pump 53-2 is connected through thetube 76-2 to a compressor 54-2. The pumping outlet of the compressor54-2 is connected to the flexible tubes 21-2 and 22-2 through the lines77-2, 78-2, 79-2, and the regulators 61-2 and 62-2. Thus, after thehigh-purity inert gas pumped from the dry vacuum pump 53-2 isre-pressurized by the compressor 54-2 and adjusted to the properpressure by the regulators 61-2 and 62-2, it is again supplied to thestatic pressure bearings of the XY table.

As mentioned above, the gas supplied to the static pressure bearingsmust be kept at the highest possible level of purity, with the leastpossible moisture and oil content. Therefore, the turbomolecular pumps,dry pumps, and compressor must be constructed to prevent entry ofmoisture or oil into the gas flow path. Also, a cold trap or filter(60-2), provided midway in the tube 77-2 connected to the pumping sideof the compressor, can be effective in trapping impurities such asmoisture or oil mixed in with the re-circulated gas, to prevent theirbeing supplied to the static pressure bearings.

This recirculation and reuse of high-purity inert gas conserves gas, andalso avoids the discharging of used inert gas into the room in which theequipment is installed, thus eliminating the possibility of having anyinert gas asphyxiation accidents.

A high-purity gas supply system 63-2 is connected to the gas circulationtubing system. This system has two functions: it fills the entire gascirculation system, including the vacuum chamber C and vacuum evacuationlines 70-2 through 75-2 and pressurization lines 76-2 through 80-2, withhigh-purity inert gas when gas circulation is initiated; and it suppliesadditional gas if, for some reason, a reduction in the flow rate of thecirculating gas occurs during operation.

It is also possible to have a single pump perform the functions of boththe dry vacuum pump 53-2 and the compressor 54-2, by assigning, to thedry vacuum pump 53-2, the function of compressing the gas to greaterthan atmospheric pressure.

In addition, instead of using a turbomolecular pump as the ultra-highvacuum pump for evacuating the column, another type of pump such as anion pump or getter pump may be used. When pooling pumps such as this areused, however, it will not be possible to construct a circulation tubingsystem in this part of the system. Also, instead of a dry vacuum pump, adiaphragm-type dry pump, or other type of dry pump may, of course, beused.

Effects of an Electron Beam Inspection Apparatus with an Improved Stage

The following effects can be brought to fruition through the use of anelectron beam inspection apparatus of the present invention equippedwith a stage such as described above.

-   a. Stable electron beam processing of a sample on a stage can be    performed using a stage of the same construction as that of a static    pressure bearing-type stage of the type normally used in the    atmosphere (a static pressure bearing support stage without a    differential pumping mechanism).-   b. It will be possible to reduce detrimental effects on the level of    vacuum in the electron beam irradiation region to an absolute    minimum, thus providing for stable processing of samples by the    electron beam.-   c. An inspection apparatus with high precision stage positioning    performance, and highly stable vacuum in the electron beam    irradiation region can be provided at low cost.-   d. Lithography apparatus with high precision stage positioning    performance, and highly stable vacuum in the electron beam    irradiation region can be provided at low cost.-   e. It will be possible to form extremely fine-featured semiconductor    circuits by using an inspection apparatus with high precision stage    positioning performance and highly stable vacuum in the electron    beam irradiation region to manufacture those circuits.    Maintenance of Vacuum in the Working Chamber (Stage Improvement 2)

In the description of the above improvement related to the maintenanceof vacuum in the working chamber (Stage Improvement 1), a prior systemhaving a stage comprising a differential discharge mechanism combinedwith static pressure bearings was described. In this system, when thestage moved, guide surfaces opposite the static pressure bearings movedback and forth between the high pressure gas atmosphere in the staticpressure bearing portion and the vacuum environment in the chamber. Whenthis occurred, during the time the guide surface was exposed to the highpressure gas atmosphere, gas was adsorbed on the surface; and during thetime it was exposed to the vacuum atmosphere gas was released, in acontinually repeating cycle. This produced a phenomenon in which thelevel of vacuum in the chamber was further degraded with each movementof the stage, making stable performance of electron beam processes suchas lithography, wafer inspection, or micromachining impossible, andthere were therefore problems with samples becoming contaminated.

It is an object of the present invention to overcome the above problemof the prior system by providing an electron beam inspection apparatushaving a stage improved as described below.

Electron Beam Apparatus with an Improved Stage

First Embodiment

A first embodiment of the apparatus is shown in FIGS. 22(A) and (B).Installed on the top surface of a Y-movable unit 5-3 of a stage 3-3 is adiaphragm 14-3 made with a large, substantially horizontal, overhang inthe +Y and −Y directions (the rightward and leftward directions of FIG.22(B)), and with a constantly-low-conductance gap closure stop 50-3disposed between the diaphragm and the top surface of an X-movable unit6-3. Also, installed on the top surface of the X-movable unit 6-3, is asimilar diaphragm 12-3 having an overhang in the +X and −X directions(the leftward and rightward directions of FIG. 22(A)), with a gapclosure stop 51-3 sandwiched between the diaphragm and the top surfaceof a stage mount 7-3. The stage mount 7-3 is attached to the floor plateof a housing 8-3 by a known method.

The gap closure stops 50-3 and 51-3 are always formed such as to closethe opening, regardless of the location into which a sample seat 4-3 ismoved. Therefore, any gas expelled from guide surfaces 6 a-3 and 7 a-3during movement of the movable units 5-3 and 6-3 will be blocked fromentering into the space C by the gap closure stops 50-3 and 51-3. Thisminimizes pressure fluctuations in the proximity of a space 24-3 (thespace in which the sample is irradiated by the electron beam).

Although not shown in FIGS. 22(A) and (B), formed in the side and bottomsurfaces of the Y-movable unit 5-3 of the stage 3-3 and the bottomsurface of the X-movable unit 6-3, around a static pressure bearing 9-3,are differential pumping channels as shown in FIG. 2. Because thesechannels are evacuated to a vacuum state, gas expelled from the guidesurfaces while the gap closure stops 50-3 and 51-3 are re-forming, ispumped mainly through these differential pumping channels. This makesthe pressure in the stage (in the spaces 13-3 and 15-3) higher than thepressure in the chamber C. Also, if in addition to discharging thespaces 13-3 and 15-3 through the differential pumping channels, separatevacuum-evacuated locations are provided, the pressure in the spaces 13-3and 15-3 can be reduced, thereby minimizing any pressure increase in theregion 24-3 near the sample. Vacuum evacuation paths 11-1.3 and 11-2.3are provided for this purpose. The pumping path 11-1.3 passes throughthe stage 7-3, and the housing 8-3, to the exterior of the housing 8-3.The discharge path 11-2.3 is formed in the X-movable unit 6-3, with itsoutlet in the bottom surface thereof.

Also, when the diaphragms 12-3 and 14-3 are installed, the chamber hasto be enlarged to prevent interference between the chamber C and thediaphragms. This can be overcome, however, by using a design or materialfor the construction of the diaphragms that will enable them to extendand retract. In this embodiment, an accordion-type flexible rubberconstruction is employed, with the ends of the diaphragms in thedirections of motion thereof fastened to the X-movable unit 6-3 (if itis a diaphragm 14-3), or to the inside wall of the housing 8-3 (if it isa diaphragm 12-3).

Second Embodiment

A second embodiment of the apparatus is shown in FIG. 23. In thisembodiment, a cylindrical diaphragm 16-3 is placed around the electronbeam irradiator 2-3 (the output end of the column), between the columnand the top of the sample S, for forming a gap closure stop. In thisconfiguration, if gas escapes from the XY stage, increasing the pressurein the chamber C, because the diaphragm interior 24-3 is partitioned offby the diaphragm 16-3, and is being evacuated through the vacuum line10-3, a pressure difference can develop between the space inside thechamber c and the diaphragm interior 24-3, and any increase in pressurein the space within the diaphragm interior 24-3 can therefore be keptsmall. As for the size of the gap between the diaphragm 16-3 and thesurface of the sample, it varies depending on the approximate pressureto be maintained in the chamber C and around the irradiator 2-3, butgenerally, a gap of from a few μm to a few mm is appropriate. Also,communication between the diaphragm 16-2 interior and the vacuum line isestablished by a known method.

Also, in some electron beam irradiation systems, high voltage on theorder of several kV is applied to the sample S, and discharge couldtherefore occur if conductive material is placed near the sample. Insuch systems, discharge between the diaphragm 16-3 and the sample S canbe avoided by making the diaphragm 16-3 of an insulator material such asceramic.

A ring member 4-1.3 placed around the sample S (wafer) is a planaradjustment component attached to the wafer seat 4-3. It is set to thesame height as the wafer, in order to form an ultra-fine gap 52-3 fullyaround the bottom edge of the diaphragm 16-3. This permits operationeven in cases where the electron beam irradiation must be performed nearthe outer edge of a sample such as a wafer. Because of this, regardlessof where, on the sample S, the point being irradiated by the electronbeam is located, the bottom edge of the diaphragm 16-3 will always forma gap 52-3 of a set size, thus ensuring that the pressure in the space24-3 around the output end of the column can be held stable.

Third Embodiment

A third embodiment, shown in FIG. 24 uses essentially the sameconfiguration as that described above (Stage Improvement 1, forimproving the maintenance of stable vacuum in the working chamber). Inthis embodiment, a diaphragm 19-3 with a built-in differential dischargestructure is provided around the electron beam irradiator 2-3 of thecolumn 1-3. The diaphragm 19-3 is cylindrical in shape, and has acircular channel 20-3 formed inside, with a pumping path 21-3 extendingupward from the circular channel. This discharge path connects to avacuum line 23-3 through an internal space 22-3. An ultra-fine gap offrom a few tens of μm to a few mm is formed between the bottom end ofthe diaphragm 19-3 and the top surface of the sample S.

In this configuration, when gas is emitted from the stage during itsmovement, raising the pressure in the chamber C, and thus urging a flowof gas into the electron beam irradiator 2-3 (output end of the column),because the diaphragm 19-3 has closed the gap between itself and thesample S, reducing the conductance therethrough to an extremely lowvalue, the flow of gas is impeded and the flow rate reduced. Inaddition, because inflowing gas is emitted through the circular channel20-3 to the vacuum line 23-3, almost no gas flows into the space 24-3around the electron beam irradiator 2-3, and the pressure at theelectron beam irradiator 2-3 can therefore be maintained constant at thedesired level of high vacuum.

The differential pumping structure provided in the diaphragm 19-3 is asshown in FIG. 19 through FIG. 21.

Fourth Embodiment

A fourth embodiment is shown in FIG. 25. A diaphragm 26-3 is providedaround a chamber C and an electron beam irradiator 2-3, for separatingthe electron beam irradiator 2-3 from the chamber C. This diaphragm 26-3is connected through a support member 29-3 (made of a material that is agood conductor, such as copper or aluminum) to a refrigeration unit30-3, which cools it to between −100° C. and −200° C. A member 27-3, forblocking thermal conduction between the chilled diaphragm 26-3 and thecolumn, is made of a material of poor thermal conductivity, such as aceramic or resin. Also, a member 28-3, made of a non-insulating ceramic,etc., is formed at the bottom end of the diaphragm 26-3, to preventdischarge between a sample S and the diaphragm 26-3.

Through such a configuration, gas molecules attempting to flow from thechamber C to the electron beam irradiator are blocked by the diaphragm26-3, and any gas that does manage to enter is frozen onto the surfaceof the diaphragm 26-3. Low pressure can therefore be maintained at theelectron beam irradiator 24-3.

For the refrigeration unit, a wide variety of refrigeration units suchas a chiller using liquid nitrogen, an He refrigeration unit, or a pulsetube-type refrigeration unit, etc. may be used.

Fifth Embodiment

A fifth embodiment is shown in FIG. 26. Provided on the two movableunits of the stage 3-3 are the same diaphragms 12-3 and 14-3 as areshown in FIG. 22. Therefore, regardless of where the sample stage 4-3 ismoved, these diaphragms, through the gap closure stops 50-3 and 51-3,will partition off the inner space 13-3 from the chamber C. In addition,formed around the electron beam irradiator 2-3, is a diaphragm 16-3 likethe one shown in FIG. 23, for partitioning off the inside of the chamberC from the electron beam irradiator 2-3 space 24-3, through a gapclosure stop 52-3. Through this configuration, when gas adsorbed duringstage movement is discharged into the space 13-3, raising the pressurein this portion, any increase in pressure in the chamber C can besuppressed, and any pressure increase within the space 24-3 can be evenfurther suppressed. In this manner, the pressure in the electron beamirradiation space 24-3 can be maintained in a low pressure state. Also,a differential pressure mechanism such as that as shown in FIG. 24 forthe diaphragm 16-3, can be used for the diaphragm 19-3. Also, by using adiaphragm 26-3 cooled by a refrigeration unit as shown in FIG. 25, thespace 24-3 can be stably maintained at an even lower pressure.

Effects of an Electron Beam Apparatus with an Improved Stage (2)

In an electron beam inspection apparatus such as described above, thefollowing effects can be obtained:

-   a. The stage system is capable of providing high precision    positioning performance in a vacuum while also making it difficult    for the pressure near electron beam irradiation position to rise.    Thus more precise processes can be performed on the sample by the    electron beam.-   b. Almost none of the gas emitted by the static pressure bearing    wafer support system can pass through the diaphragm into the region    of electron beam irradiation. This makes the vacuum at the location    being irradiated by the electron beam more stable.-   c. Because it is harder for discharged gas to penetrate into the    region in which electron beam irradiation region is taking place, it    is easier to hold stable vacuum in this electron beam irradiation    region.-   d. The vacuum chamber interior is partitioned by low conductance    barriers into three chambers: an electron beam irradiation chamber,    a static pressure bearing chamber, and intermediate chamber. Also,    the vacuum evacuation system is configured such that the pressures    in the respective chambers increase from chamber to chamber in the    following order: electron beam irradiation chamber (lowest    pressure), intermediate chamber (intermediate pressure), and static    pressure bearing chamber (highest pressure).

Pressure changes induced in the intermediate chamber are further reducedby a diaphragm, and pressure changes induced in the electron beamirradiation chamber are reduced by an additional step by anotherdiaphragm, ultimately reducing pressure fluctuations to a level at whichthey are virtually a non-problem.

-   e. Increased pressure during stage motion can be kept at a low    level.-   f. Increased pressure during stage motion can be kept at an even    lower level.-   g. Because an inspection apparatus having both highly accurate stage    positioning performance and stable vacuum in its electron beam    irradiation region can be realized, an inspection apparatus    featuring high inspection performance with no sample contamination    problems can be provided.-   h. Because a lithography system having both highly accurate stage    positioning performance and stable vacuum in its electron beam    irradiation region can be realized, a lithography system featuring    high inspection performance with no sample contamination problems    can be provided.-   i. It will be possible to form extremely fine semiconductor circuits    through the use of apparatus having highly accurate stage    positioning performance and stable vacuum in the electron beam    irradiation region.    Improving Throughput (Electron Optics With Multiple Optical Systems    (Columns)

In the above embodiment of the electron optical system of the inspectionapparatus, an electron beam emitted from a single electron source waspassed through an aperture plate having multiple apertures in order toform multiple beams (multibeam beamlets). These multiple beams were usedto improve throughput in a column with a single optical system forperforming wafer inspections. In the present aspect of the invention,however, multiple columns (optical systems) are provided. These multipleoptical systems make it possible to inspect a number of different areas(and thus a larger total area) at the same time, to thus provide anelectron beam optical apparatus capable of an even greater improvementin throughput.

First Embodiment of Electron Optical Apparatus With Multiple OpticalSystems (Columns)

In this first embodiment, as shown in FIG. 28, four electron opticalsystems (columns) 1 a-4, 1 b-4, 1 c-4 and 1 d-4 (with the maximumdiameters 60 a-4, 60 b-4, 60 c-4 and 60 d-4, respectively) are used toscan the surface of a wafer W for inspection. The four optical systemsare placed over the wafer along a line perpendicular to the direction ofmotion 21-4 of a stage 48-4 (i.e., the wafer W).

Each of the electron optical systems 1 a-4, 1 b-4, 1 c-4, and 1 d-4,which all have the basic configuration shown in FIG. 7 (b), comprises

-   -   an electron gun 1-4;    -   a condenser lens 2-4;    -   a multi-aperture plate 3-4;    -   an aperture stop 4-4    -   a condenser lens 5-4;    -   an ExB separator 7-4;    -   electrostatic deflectors 6-4 and 8-4;    -   an objective lens 10-4;    -   magnification lenses 12-4 and 13-4;    -   a detector aperture plate 14-4;    -   a detector 15-4;    -   a deflector 20-4;    -   a stage 48-4;    -   a controller 50-4; and    -   a display 52-4.

FIG. 27( a)) is a top view showing typical positional relationshipsbetween the primary electron beam irradiation system (primary opticalsystem) and the secondary electron beam detection system of the opticalsystem 1 a-4 (one of the four optical systems in FIG. 28). Here, theoutline 60 a-4 represents the maximum diameter of the primary electronbeam irradiation system, and each of the equally spaced circles on thediameter line of the maximum diameter outline 60 a-4 represents one ofthe multi-aperture regions 16-4 of a primary electron beamlet that haspassed through one of the apertures 17-4 of the multi-aperture plate3-4. The line 18-4 represents the optical axis of the secondary electrondetector. The secondary electron beamlet emitted from the wafer byelectron beam irradiation of the multi-aperture region 16-4 is deflectedby the ExB separator 7-4. Then it passes along the optical axis 18-4,where it is magnified, and is then detected by its correspondingdetector element in the detector 15-4. Corresponding positionalrelationships between the primary electron beam irradiation system andthe secondary electron detection system are defined so that each beamletwill be detected by a specific one of the multiple detector elements ofthe detector 15-4. As will be understood more clearly from the drawings,each of the multi-aperture regions 16-4 (i.e. each of the multipleapertures 17-4), is paired with a specific one of the multiple detectorelements, such as to prevent the occurrence of crosstalk between thebeamlets along the way.

Each of the other optical systems that make up the total electronoptical system of the present embodiment (1 b-4, 1 c-4, and 1 d-4) alsohas a primary electron beam irradiation system, an ExB separator, and asecondary electron detection system as described above. The controller50-4 and stage 48-4 can be used in common by all of the optical systems.The secondary electron beam image processing circuits, etc., constitutesome of the functions of the controller 50-4. If necessary, however,rather than imbedding these circuits entirely within the controller,they may be provided in the individual optical systems. These multipleoptical systems 1 a-4, 1 b-4, etc. are all placed in a line over thesame wafer such that each irradiates a different region of the waferwith primary electrons, and each detects secondary electrons from itsown region.

To prevent optical systems from interfering with each other, thesecondary optical axes 18-4 of the optical systems are aligned with thedirection of motion of the stage 21-4 (i.e. perpendicular to the linealong which the optical systems are placed) such that adjacent axes aredirected away from the line in opposite directions. This arrangementcauses each axis to be aligned with a multiple aperture region 16-4 andan element of the detector 15-4, and perpendicular to the direction ofstage motion 21-4.

With optical systems placed as shown in FIG. 28, if we assume a maximumdiameter of 40 mmφ for a primary electron beam irradiation system, itwould be possible to place five optical systems over an 8-inch(approximately 20-cm) wafer. Since this would put much of the peripheralportions of the optical systems off the wafer, however, the practicallimit is four optical systems. However, if a maximum diameter of 30 mmφis feasible, this number can be increased to around six.

Next, the operation of this electron beam optical apparatus will bedescribed. A single primary electron beam emitted from the electron gun1 in each of the optical systems 1 a-4 through 1 d-4 is converged by thecondenser lens 2-4 to form a crossover image at the aperture stop 4-4.Along the way, the primary electron beam of each optical systemilluminates the multi-aperture plate 3-4, which forms seven beamlets (inthis embodiment) as they pass through the apertures 17-4. Images ofthese multiple beamlets are formed on a main surface 11-4 of the ExBseparator 7-4 by the condenser lens 5-4, and are again formed asdemagnified images on the wafer W, by the objective lens 9-4. At thistime, seven beam spots on the wafer are irradiated for each opticalsystem (see FIG. 28), and secondary electrons are emitted from each ofthese spots. The electrostatic deflectors 6-4 and 8-4 deflect thebeamlets perpendicular to the direction of stage motion 21-4, over anarea slightly larger than the separation between adjacent beamlets. Thisdeflection enables the irradiated spots on the wafer to be scanned alongthe direction of the row of beams without interruption. As the beams arebeing scanned in this manner, the stage 48-4 is controlled in continuoussynchronous motion, through successive periodic sweeps of prescribedwidth in the direction of stage motion 21-4, thus making it possible toscan the entire inspection surface of the wafer. For example, if weassume that the width that can be scanned in the direction of the row ofbeams by the four electron optical system is 2 mm, then in 20consecutive sweeps of the stage, it will be possible to evaluate an area160×160 mm in size (with all four optical systems).

The multiple secondary electron beamlets emitted from the irradiatedspots are accelerated substantially perpendicularly upward to arrive atthe ExB separator 7-4, where they are deflected at a prescribed anglewith respect to the optical axis 55-4 by ExB fields that exist in theseparator, and continue along the optical axis 18-4 of the secondaryoptical system. The separation between these multiple secondary electronbeamlets is increased by magnification lenses 12-4 and 13-4, and theypass through the detector aperture plate 14-4 to be detected by themultiple detector elements 15-4. At the same time, mispositioning of thesecondary electron beamlets due to deflection of the primary electronbeam by the deflectors 6-4 and 8-4 is offset by the correction deflector19-4. In other words this correction is made such that regardless ofwhat occurs during the primary electron beam scan, each of the secondaryelectron beamlets will always pass through its own particular apertureof the detector aperture plate 14-4 to be detected by its own detectorelement behind that aperture. The multiple detector elements 15-4 outputsignals indicative of the intensity of the secondary electron beam tothe controller 50-4. The controller 50-4 receives the output signalsfrom the detector elements 15-4 serially, in synchronization withprimary electron beam-deflector control and stage 48-4 motion control,and ultimately obtains therefrom, an image of secondary electron beamintensity distribution over the entire inspection surface of thesemiconductor wafer W.

The controller 50-4 compares actual detected secondary electron beamimages with the secondary electron beam image of a defect-free wafer,that is stored in memory in advance, in order to automatically detectdefective portions. Also, when a wafer has a large number of identicaldies, defective portions may also be detected by comparing the detectedimages of the inspected dies against each other.

When performing inspections, in addition to displaying the detectedimages on the display 50-4, portions judged defective can be marked withdefect indicators. Operators can then make a final check and evaluationof the wafers W to determine whether it actually has defects.

Specific examples of defect inspections are shown in FIGS. 30 though 32.First, shown in FIG. 30 is a die 31-4 that was the first one inspected,and another die 32-4 that was the second on inspected. If the image of aseparate die inspected third is judged to be similar or identical to thefirst die 31-4 inspected first, then the portion 33-4 of the secondinspected die 32-4 is judged defective, and the defective portion of itis inspected.

FIG. 31 shows an example of measurement of the line width in a patternformed on a wafer. The signal 36-4 represents the actual secondaryelectron intensity signal obtained when an actual wafer pattern 34-4 wasscanned in a direction 35-4. The line 37-4 indicates a threshold levelcalibrated in advance for the signal 36-4. The width (38-4) of theportion of that signal that continuously exceeds the threshold is thenmeasured as the line width of the pattern line 34-4. When the measuredline width of a pattern falls outside of a prescribed range, thatpattern is judged to be defective.

FIG. 32 shows an example of measurement of electrical potential contrastof a pattern formed on a wafer W. An axially symmetrical electrode 39-4(FIG. 32) is placed in the configuration shown in FIG. 27( b), betweenthe objective lens 9-4 and the wafer. A voltage is applied to thiselectrode 39-4 such as to place it at a potential of −10 V, for example,with respect to the wafer (which is at 0 V potential). The −2 Vequipotential plane will be assumed to have the form indicated by thecurve 40-4. Also, the potential on the pattern 41-4 is assumed to be −4V, and that on the pattern 42-4 is assumed to be 0 V. Under theseconditions, since the secondary electrons emitted from the pattern 41-4(at -4 V) will have an upward velocity equivalent to 2 eV of kineticenergy at the equipotential plane 40-4, they will pass through thispotential barrier 40-4, and escape from the electrode 39-4 along thetrajectory 43-4, to be detected by the detector 15-4.

Secondary electrons emitted from the 0 V pattern 42-4, however, cannotovercome the −2 V potential barrier, and will therefore return to thewafer surface along the trajectory 44-4, without being detected. Thedetected image of the pattern 41-4, then, will be clear, and that of thepattern 42-4 will be dark. In this manner, potential contrast isobtained. If the brightness of the detected image and potential arecalibrated in advance, the pattern potential can be measured via thedetected image. Also, defective portions of a pattern can be found byanalyzing the potential distribution of the pattern.

A blanking deflector 20-4 is provided is the configuration of FIG. 27(b). This deflector can be used to generate a pulsed beam with shortpulse duration by causing the deflector 20-4 to deflect the primaryelectron beam away from the aperture portion of the aperture stop 4-4 ina repeating cycle of a prescribed period, such that the beam will passthrough the aperture for only a short portion of each cycle, and will beblocked the rest of the time. If such a narrow-pulse pulsed beam is usedto measure the potentials on a wafer, as described above, time domainanalyses of device operation can be performed with high time resolution.In other words the present electron optical system can be used as amultibeam EB tester.

In this first embodiment, the stage 48-4 operation has few returnoperations. This results in wasted time associated with stage movement(wasted time that can be reduced).

Second Embodiment

The electron beam optical apparatus of the second embodiment is relatedto a configuration in which multiple optical systems are arranged in a2-row, m-column (m>1) pattern. FIG. 29 is a top view showing an examplein which six optical systems are configured in a 2-row, 3-columnpattern. The specific constituent elements of the electron opticalsystem and the individual optical systems are substantially the same asin the first embodiment, and they are therefore assigned the samereference symbols here, they will not be described in detail.

In FIG. 29, the maximum diameters of the six optical systems of theprimary electron beam irradiation system are indicated by the outlines60 a-4 through 60 f-4. To prevent these multiple optical systems frominterfering with each other, the optical axes 18-4 of the secondaryoptical systems (the paths of the secondary electron beams) are lined upin the row direction, pointing toward the outer edge of the wafer. Thepreferred number of columns (m) is around 3 or 4, but two columns, orfour, may also be added, outside of these.

The multi-aperture regions 16-4 and the detector 15-4 are configured ina 3-row, 3-column array as the maximum number of beamlets and detectorelements can be included in one optical system without encounteringexcessive aberration. The stage 48-4 is operated in a repeating seriesof step movements to move it within the horizontal plane. Scanning isperformed the same way as in the first embodiment.

In this second embodiment, the number of optical systems (columns) isincreased, and each optical system has more beamlets and detectorelements, and it therefore further improves semiconductor waferinspection process throughput.

(Effects of the Multiple Optical System (Column) Electron OpticalSystem)

As described in detail above, provided according to the multiple opticalsystem electron beam optical apparatus of the present invention, are aplurality of optical systems that are capable of performing irradiationof primary electrons and detection of secondary electrons separately,for inspecting different regions on the sample. This provides anexcellent effect in that throughput can be greatly improved whilemaintaining high resolution.

Alignment of the Electron Optics of the Multibeam Inspection Apparatus

The axes of the optical systems of the multibeam inspection apparatusdescribed above require alignment. The electron beam inspectionapparatus of the present invention has an alignment system for thispurpose. That system is described below.

(Primary Optical System Alignment)

FIG. 33 shows an electron optical system that will be used to describealignment according to the present invention. The configuration andoperation of this system are essentially the same as in the multibeaminspection apparatus described above, and their descriptions willtherefore not be repeated here.

As in the above systems, this electron optical system has a primaryoptical system comprising an electron gun 1-5; a condenser lens 2-5; ademagnification lens 6-5; two electrostatic deflectors 5-5 and 12-5; anaxially symmetrical electrode 11-5; and an objective lens 9-5. It alsohas a secondary optical system comprising two magnification lenses 14-5and 15-5. Alignment of the primary optical system will be describedfirst. The alignment we will now be discussing consists of aligning theaxis lines of the multibeam beamlets with the optical axis of theoptical system.

Axial alignment of the lenses of the primary optical system (i.e., theaxial alignment of the condenser lens 2-5, the demagnification lens 6-5,and objective lens 9-5) basically involves making adjustments so thatthe amount of movement of at least two of the beam positions on thesurface of the sample caused by a small change in the excitationvoltages of the lenses will be the same. Here, the two beams used forthe alignment are equidistant from the multibeam center (for example,two beamlets positioned on a circle centered on the point indicated onthe drawing).

The axial alignment of the objective lens 9-5 can be performed asfollows: First, as shown in FIG. 34, marks (21-5, 22-5, . . . 2 n-5)made up of combinations of X-lines and Y-lines (indicating irradiationreference locations for each beamlet) are provided at locations on thesurface of a sample 10-5 at which the multiple primary electron beamlets(indicated by the black dots) are imaged, and the beam focus conditions(the lens excitation voltages at which the beam is properly focussed)are measured for each mark. For these measurements, the signal contrastwhen multiple primary beamlets are scanned in the X direction, and thesignal contrast when the beams are scanned in the Y direction aremeasured at a minimum of three objective lens excitation voltages, andthe results plotted in a graph of change in contrast vs. excitationvoltage. The convergence condition can then be determined from the graphobtained: For example, if V_(ox) is the excitation voltage at which thecontrast in the X direction is the greatest, and V_(oy) is theexcitation voltage at which the contrast in the Y direction is thegreatest, then the convergence condition would be (V_(ox)+V_(oy))/2. Theaxially aligned condition for the objective lens 9-5 is then determinedas the condition at which the least difference exists between theconvergence conditions of at least two electron beamlets (e.g., twobeamlets diametrically opposite each other on a circle centered on theoptical axis). In other words, when the difference between convergenceconditions is minimum, the beams will pass through the objective lens atpoints at which the difference in the distances, from the optical axis,of the two beams, is minimum.

Performing axial alignment of the multibeam electron optical system asdescribed above makes it possible to use multiple electron beams toincrease the throughput of various inspection processes (defectinspections, critical dimension checks, etc.) without sacrificingaccuracy.

(Secondary Optical System Alignment)

Alignment of the secondary optical system will now be described. FIG. 35shows the same kind of electron optical system as described above. Asshown, the secondary optical system of this electron optical systemcomprises a first magnification lens 9-6, a second magnification lens10-6, a multi-aperture plate 11-6, a detector 12-6, a first deflector19-6, a second deflector 20-6, and a crossover aperture stop 21-6. Thesecondary optical system axial alignment described will be for the casein which this aperture stop 21-6 is placed at a crossover locatedbetween the second magnification lens 10-6 and the detectionmulti-aperture plate 11-6.

In the system of FIG. 35, the signal from a scan signal generator 22-6is superimposed on a deflection signal from a deflection signalgenerator 23-6, and the resulting signal is applied to the two-stagedeflector made up of the first deflector 19-6 and the second deflector20-6. These two deflector stages 19-6 and 20-6, which are placedorthogonal to the optical axis, have two alignment modes: one mode inwhich they are axially aligned with the second magnification lens 10-6,and another mode in which they are axially aligned with the aperturestop 21-6. In the two modes, the signal strength ratio of the signalsapplied to the two deflectors (19-6 and 20-6) from the scan signalgenerator 22-6 and the deflection signal generator 23-6 are set topredetermined values for the alignment mode being used, and thedeflectors 19-6 and 20-6 are then controlled according to this ratio.For example, to perform axial alignment with the second magnificationlens 10-6, the deflector 19-6 output might be set to 1 and the deflector20-6 output to -1.5; whereas for axial alignment with the aperture stop21-6, the signal strength ratio would be determined such as to put thecenter of the principal plane of the second magnification lens 10-6 atthe center of deflection.

The image processor 14-6 forms images in synchronization with thescanning of the electron beamlets over the aperture stop 21-6 by thedeflectors 19-6 and 20-6. The scan signal is applied to the deflectors19-6 and 20-6, and also to an image forming circuit in the imageprocessor 14-6. When a signal from one of the elements of the multibeamdetector 12-6 is applied to the image processor 14-6 as image data, ofthe detector element addresses corresponding to the scan signal of theimage forming circuit of the image processor 14-6, the only addressesfor which strong signals will be input to the image processor 14-6 fromthe detector 12-6 are those addresses that correspond to electron beamsthat have passed through the aperture stop 21-6. Thus when the axes arealigned, an aperture stop image 24-6 will be formed as shown in FIG. 36(a).

If the axes are misaligned, however, the signal aperture stop image 25-6will be formed when the scan signal x,y address is strong-offset fromthe 0,0 location, as shown in FIG. 36( b). When this is the case, theoutputs supplied to the deflectors 19-6 and 20-6 from the deflectionsignal generator 23-6 are changed as required to cause those deflectorsto deflect the secondary electron beam B2 so that the aperture imagecoincides with the address at which the X and Y scan signals are both at0 volts, as shown in FIG. 36( a). When this is accomplished, the axialalignment is complete. In other words, the beam now passes through thecenter of the aperture stop. In this configuration, the deflectionsignal generator 23-6 includes the deflection signal generating device,as well as a device for changing the output of the deflection signalgenerating device and supplying it to the deflectors. This alignmentprocedure can be performed automatically, entirely without humanintervention.

In this electron optical system, the following effects can be brought tofruition by performing the axial alignment as described above:

Axial alignment with respect to the aperture stop can be performedautomatically.

Because the deflectors used for normal beam scanning are also used foraxial alignment, only half as many deflectors are required.

It makes it possible to perform axial alignment of a multibeam system.

Because a secondary optical system aperture stop is provided between theExB separator and the multibeam detector, the positioning of this stopcan be determined independently of the primary optical system aperturestop.

(Axial Alignment of the Wien Filter/ExB Separator)

The ExB separator used in the electron optical system of the inspectionapparatus described above is configured to form mutually orthogonalelectric and magnetic fields in a plane perpendicular to the surface ofthe sample. The operation of the ExB separator is such that allelectrons for which the relationships between the electric and magneticfields and the energy and speed of the electrons satisfy a given set ofconditions, are sent straight through the separator without beingdeflected; and all other electrons are deflected. Within the ExBseparator structure, there exists a region in which the distribution ofelectric and magnetic fields is uniform; and a region in which it isnot. Therefore in order to perform defect inspections with good accuracyusing an electron beam inspection apparatus with multiple beamlets, theaxis of each of those beamlets has to be aligned with the axis of theExB separator. That is, the region of the ExB separator over which thedistribution of electric and magnetic fields is uniform has to bedetermined, and adjustments made so that each of the electron beamletspasses through that region.

When an ExB separator is used in a multibeam electron beam inspectionapparatus, however, the region, of the ExB separator, over which theelectric and magnetic fields are uniform, and the region, of the ExBseparator, through which the multiple beams pass,

both encompass areas of about the same size. Therefore, if alignment ofelectron beamlets with the axis of the ExB separator is inadequate, someof the beamlets may fall outside of the region of uniform electric andmagnetic field distribution. This can degrade the beam characteristics,resulting in greater distortion at the edge of the field, and increasedblurring of the image.

The present invention provides a method for performing axial alignmentof multiple beams with the ExB separator, for the purpose of eliminatingthe blurring and distortion of the image due to the use of an ExBseparator in the multibeam electron beam inspection apparatus. Alignmentof multiple beams with the axis of the ExB separator is described below.

The optical system of the electron beam inspection apparatus 1 shown inFIG. 37 has essentially the same configuration as that of the inspectionapparatus described above. It comprises a primary optical system 10-7, asecondary optical system 30-7, a detector 40-7, and an X-Y stage 80-7for moving an inspection sample in the X and Y directions. A primaryoptical system 10-7, which is the optical system used to irradiate thesurface of a sample (i.e., a wafer W), comprises

-   -   an electron gun 11-7, for emitting an electron beam;    -   an electrostatic lens 12-7, for converging the electron beam        emitted by the electron gun;    -   a first multi-aperture plate 13-7 having formed therein, a        plurality of small openings (in this embodiment, 8 openings 13        a-7 through 13 h-7), arranged in a straight line);    -   an electrostatic deflector 14-7;    -   an electrostatic demagnification lens 15-7, for deflecting        multiple beamlets that have passed through the first        multi-aperture plate 13-7;    -   an electrostatic deflector 16-7 for deflecting the multiple        beamlets;    -   an ExB separator 17-7;    -   an electrostatic objective lens 18-7; and    -   an axial alignment device 19-7 for performing axial alignment of        the electron beams;

all of which, as shown in FIG. 37, are arranged in the above listedsequence with the electron gun 11-7 at the top, such that the opticalaxis A of the electron beam emitted by the electron gun is perpendicularto the surface of a sample S. Also, formed in a straight line on acathode within the electron gun, are a plurality of pointed tips (inthis embodiment, for example, 8 tips 11 a-7 through 11 h-7, as shown inFIG. 37).

The secondary optical axis 30-7 comprises

-   -   two electrostatic magnification lenses (31-7 and 32-7) placed        along an optical axis B that slants away from the optical axis A        near the ExB separator 17-7 of the primary optical system 10-7;        and    -   a second multi-aperture plate 33-7 having a plurality of small        openings (in this embodiment, 8 openings 33 a-7 through 33 h-7),        formed therein such as to match the number and arrangement of        the apertures of the first multi-aperture plate 13-7.

The detector 40-7 has a separate detector element 41-7 for each of theapertures of the second multi-aperture plate 33-7. Each of the detectorelements 40-7 is connected through an amplifier 42-7 to an imageprocessor 43-7. In addition, the same signal as that applied to theelectrostatic deflector 16-7 is also applied to the image processor43-7.

All of the above constituent elements of the system may be implementedusing commonly known components, and their construction will thereforenot be described in detail.

Next, the operation of an electron beam inspection apparatus 1-7configured as indicated above will be described. Electron beams areemitted in 8 directions from the plurality of tips (11 a-7 through 11h-7) on the cathode of the single electron gun 11-7. The emittedelectron beam C is converged by the electrostatic lens 12-7, to form acrossover C1. The electron beam C that is converged by the electrostaticlens 12-7 irradiates the first multi-aperture plate 13-7, passingthrough a plurality of small apertures (apertures 13 a-7 through 13 h-7,formed in a straight line in the X direction, for example) in the firstmulti-aperture plate 13-7, to be formed thereby into 8 multibeambeamlets. These electron multibeam beamlets are demagnified by theelectrostatic demagnification lens 15-7 and projected at positionsindicated by the points 50-7. After being converged at the points 50-7,the beamlets are again converged at the sample W by the electrostaticobjective lens 18-7. The multibeams leaving the first multi-apertureplate 13-7 are deflected by the electrostatic deflector 16-7 (placedbetween the electrostatic demagnification lens 15-7 and theelectrostatic objective lens 18-7) such as to simultaneously scan thebeamlets across the surface of the sample W.

Thus 8 spots on the sample W are irradiated by the 8 converged beamlets.Secondary electrons emitted from these 8 spots are attracted andnarrowly converged into secondary electron beamlets by the electricfield of the electrostatic objective lens 18-7. The converged secondaryelectron beamlets are then deflected by the ExB separator 17-7 to injectthem into the secondary optical system. Secondary electron images areformed at the points 51-7, which are closer to the electrostaticobjective lens 18-7 than are the points 50-7. The reason for thisdifference is that the energy of each secondary electron beamlet (only afew eV) is very small in comparison to the 500 eV energy possessed byeach primary electron beamlet. The thus-imaged secondary electrons arepropelled along the secondary optical axis B and injected into theelectrostatic magnification lenses 31-7 and 32-7. The secondaryelectrons passed through these magnification lenses are imaged at thelocations of the plurality of apertures (33 a-7 through 33 h-7) of thesecondary multi-aperture plate 33-7. The electrons passed through theseapertures are detected by corresponding detector elements 41-7 of thedetector 40-7.

When this occurs, the secondary electrons emitted from the sample W bythe electron beamlet that passed through the aperture 13 a-7 of thefirst multi-aperture plate 13-7, pass through the aperture 33 a-7 of thesecond multi-aperture plate 33-7; the secondary electrons emitted fromthe sample S by the electron beamlet that passed through the aperture 13b-7 of the first multi-aperture plate, pass through the aperture 33 b-7of the second multi-aperture plate 33-7; the secondary electrons emittedfrom the sample S by the electron beamlet that passed through theaperture 13 c-7 of the first multi-aperture plate, pass through theaperture 33 c-7 of the second multi-aperture plate 33-7, and so on. Inother words, secondary electrons emitted from the surface of the sampleby primary electron beamlets pass through apertures in the secondmulti-aperture plate 33-7 that correspond to respective apertures of thefirst multi-aperture plate 13-7, to be injected into the respectivecorresponding detector elements 41-7.

Each of the respective detector elements 41-7 converts its detectedsecondary electrons into an electrical signal representative of thesecondary electron intensity. Each of these electrical signals output bythe detector is then amplified by an amplifier 42-7, and input to theimage processor 43-7, where it is converted to image data. The imageprocessor 43-7 also receives the scan signal used to deflect the primaryelectron beams. Based on the information provided by the both theelectron intensity and scan signals, the image processor 43-7 can createan image of the surface of the sample W. By then comparing this detectedimage with a reference standard pattern, defects on the surface of thesample W can be detected.

Thus in the above system, electron beamlets passed through the aperturesof the first multi-aperture plate 13-7 are converged at the surface ofthe sample W, causing secondary electrons to be emitted from the sampleW, and detected by the detector elements 41-7. In such a system it isespecially important to minimize the effects of three types ofaberration: distortion, field curvature aberration, and fieldastigmatism caused by the primary optical system.

The sample w is scanned in two dimensions by applying scan signals tothe electrostatic deflector 16-7 and the magnetic field of the ExBseparator 17-7, and a scanning electron microscope signal is displayedin the display of the image processor 43-7. A mark 20-7 (the + symbol inFIG. 37) is provided on the surface of the sample W This size of thismark 20-7 is 5 microns, while the mutual separation between the spots atwhich the 8 electron beamlets are imaged is on the order of 100 microns.Thus the mark 20-7 can be scanned with a single beamlet, and its imagedisplayed by the image processor 43-7.

A method for performing axial alignment of the multiple electron beamswill now be described. The axial alignment device 19-7 is used to alignthe electron beams with the axis of the ExB separator 17-7. First, theposition of the X-Y stage 80-7 is determined such that, of the 8electron beamlets, only the beamlet formed by the right-most aperture 13h-7 of the first multi-aperture plate 13-7 will scan the mark 20-7 onthe sample. Then, scan signals are applied to the magnetic fields theelectrostatic deflector 16-7 and the ExB separator 17-7 as required totwo-dimensionally scan the mark 20-7. As the scan proceeds, the detectorelement 41-7 detects, and the image processor 43-7 displays, an image ofthe mark 20-7. During scanning, the voltage applied to the ExB separator17-7 is periodically varied between a reference voltage and a voltageequal to the reference voltage+10 volts. The positional displacementmagnitude 44-7 corresponds to the amount of deflection, of the beamletpassed through the aperture 13 h-7, by the ExB separator 17-7 (due tothe change in the deflection voltage applied thereto). The value of thispositional displacement magnitude 44-7 is stored in memory.

Next, the position of the X-Y stage 80-7 is determined such that onlythe beamlet formed by the left-most aperture 13 a-7 of the firstmulti-aperture plate 13-7 will scan the mark 20-7. The mark 20-7 isscanned, the result of the scan is detected by the detector element41-7, and the mark 20-7 image is displayed by the image processor 43-7.The electron beamlet formed by the aperture 13 a-7 is on the oppositeside of the optical axis A of the primary optical system 10-7 from thatof the above beamlet formed by the aperture 13 h-7, and is separatedfrom the optical axis A by the same distance. The locations of these twobeamlets, then, are separated by the maximum possible distance. With thebeamlets in this state, voltage applied to the ExB separator 17-7 isperiodically varied between a reference value and a value equal to thereference value+10 volts. Here too, the image processor 43-7 displaysthe images of two marks separated from each other, this time by apositional displacement magnitude 44′-7. This positional displacementmagnitude 44′-7 corresponds to the amount of deflection of the beamletpassed through the aperture 13 a-7 by the ExB separator 17-7 (due to thechange in the deflection voltage applied thereto). The value of thispositional displacement magnitude 44′-7 is also stored in memory.

In addition, the voltage applied to the axial alignment device 19-7 isset to a number of different values, and for each new setting, the aboveoperation with the beamlets that are passed through the apertures 13 h-7and 13 a-7 is repeated, to obtain values of positional displacement 44-7and 44′-7 for each voltage setting.

Then, the axial alignment device 19-7 voltage that yields the leastdifference between the values of positional displacement 44-7 and 44′-7is determined, and the voltage is fixed at that value, thus completingthe axial alignment for electron beamlets incident to the ExB separator.In this manner, multiple electron beamlets can be placed within thatregion of the ExB separator within which the magnetic and electricfields are uniform.

As a separate method for axial alignment of the multiple beamlets,instead of using beamlets arranged in a straight line, a plurality ofbeamlets (four in this embodiment) may be formed by providing aplurality of apertures (also four in this embodiment) in the firstmulti-aperture plate 13-7 and the second multi-aperture plate 33-7,respectively, at positions around and equidistant from, the optical axisA of the primary optical system, and the optical axis B of the secondaryoptical system, respectively. Because all four beamlets will then beseparated from the optical axis A by the same distance, when the beamsare properly aligned, a given change in the voltage applied to the ExBseparator 17-7 should produce the same amount of positional displacementfor each of the four beamlets. To accomplish this, an axial alignmentoperation must be performed individually for four of the eight beamlets.

As an alternative method, a mark may provided at each of the four pointsupon which the four beamlets are incident, the marks displayed on fourmonitors in the image processor 43-7, and the positional displacementvalues 44-7 and 44′-7 for all four beamlets measured at the same time.

Also, instead of using the image processor 43-7 to measure thepositional displacement of the mark 20-7, the measurement may beperformed automatically, and the alignment also performed by computercontrol. When this is done, however, better results will be obtained byusing line-and-space patterns in the X and Y directions for the mark20-7, rather than the + signs used above. In the electron beaminspection apparatus as described above, the following effects can bebrought to fruition by performing the above axial alignment:

-   (1) The individual multibeam electron beamlets can be stably    positioned within a region having uniform distribution of the    electric and magnetic fields of the ExB separator, such that all    beamlets can be tightly converged.-   (2) If the individual multibeam beamlets are arranged in a straight    line, a satisfactory axial alignment can be performed by aligning    only two beamlets positioned symmetrically about the optical axis of    the primary optical system.-   (3) It is possible to determine whether there is sufficient margin    within the region of uniform distribution of the electric and    magnetic fields of the ExB separator to allow passage therethrough    of all of the multibeam beamlets.-   (4) Axial alignment of electron beamlets incident to the ExB    separator can be performed in a wobbler-like operation in which    symmetrical positions of the magnetic fields of the lenses are    found, and the beamlets moved in the directions of these positions.    Alignment of Multibeam Beamlets with Multi-aperture Plates.

To obtain high brightness, the electron gun used in the multibeam systemmust emit a highly directive beam. To obtain sufficiently intensebeamlets from a highly directive beam irradiating the multi-apertureplate, however, the large area of high beam intensity emitted from theelectron gun must accurately overlay the aperture locations on themulti-aperture plate. Also, to efficiently evaluate the pattern of aninspection sample, the direction, on the sample surface, in which themultiple beamlets incident thereto are aligned, must be accuratelyaligned with the axis coordinates.

In light of the above, the present invention is configured to ensurethat the high intensity region of the beam from the beam emission sourceincident to the multi-aperture plate, will be properly aligned with theaperture locations of the multi-aperture plate, and that the arrangementand orientation the multiple beamlets obtained therefrom will beaccurately aligned with the orientation of the pattern on the surface ofthe sample.

An embodiment of this invention is described below. Shown in FIG. 38 isan electron gun 1-8. The electron gun 1-8 comprises a cathode 3-8, aWehnelt electrode (beam converging electrode) 5-8, and an anode 7-8. Thecathode 3-8 is made of monocrystalline LaB₆ formed in the shape of atruncated cone having a plurality of tips (i.e., beam-emitter tips,formed side by side along a circle on an end surface of the cathodefacing the anode). When a negative bias voltage (negative with respectto the cathode) applied to the Wehnelt electrode 5-8 is increased (mademore negative), a beam crossover 9-8 formed by the electron gun isshifted nearer the cathode, and the path of the beams emitted from thebeam-emitter tips arranged along a circle on an end surface of thecathode shift away from the path 11-8 (indicated by the dotted line inthe drawing), toward the path 13-8 indicated by the solid line.Conversely, if the negative bias voltage on the Wehnelt electrode 5-8 isdecreased (made less negative), the beam path shifts away from the solidline, and toward the dotted line.

After emerging from the normally grounded anode 7-8, the beam isconverged by a condenser lens 15-8 to form a crossover 17-8. Provided onthe electron gun side of the crossover 17-8 is a multi-aperture plate19-8 having multiple apertures therein for shaping the irradiating beaminto multibeam beamlets. When the multi-aperture plate 19-8 isirradiated with beams from the electron gun 1-8, the center 01 of themulti-aperture plate, and a point O2 central to all of the beamletsincident thereto, are axially aligned by an axial alignment coil 21-8.That is, if the center O1 of the multi-aperture plate and the point O2central to all of the beams incident thereto are misaligned as shown inFIG. 39, then the small apertures a1-a7 of the multi-aperture plate 19-8will not be properly aligned with the large regions of high beamintensity b1-b7 of the beams irradiating the multi-aperture plate.Consequently, there will be differences in beam intensity between themultibeam beamlets emerging from the multi-aperture plate 19-8.Therefore, the axial alignment coil 21-8 is used to parallel-shift allof the beams as required to adjust the point O2 (which is central to allof the beams incident to the multi-aperture plate 19-8) into alignmentwith the center O1 of the multi-aperture plate, such that the beamletsemerging from the apertures a1-a7 of the multi-aperture plate will allbe of uniform beam intensity.

FIG. 40 shows the relationships between the multibeam beamlets that haveundergone the above alignment adjustment by the alignment coil 21-8, andthe corresponding apertures of the multi-aperture plate. As can be seenfrom this drawing, it is not enough to simply align the point O2 centralto all of the beams incident to the multi-aperture plate 19-8 with thecenter O1 of the multi-aperture plate. In the state shown in FIG. 40,for example, the high beam intensity regions b1-b7 (regions of high beamintensity of the beams incident to the multi-aperture plate) are notproperly aligned with the aperture positions a1-a7 of the multi-apertureplate 19-8, and it is apparent that the alignment at this point isinadequate. That is, there is radial displacement 41-4, and an azimuthdisplacement 43-4 between the apertures a1-a7 of the multi-apertureplate and the high-beam-intensity regions b1-b7, which in some casesaligns the apertures with regions of not-so-high beam intensity c1-c7bof the beams incident to the multi-aperture plate. Therefore, thepresent invention has provisions for making fine adjustments in theradial and azimuth directions as required to properly align regions ofhigh beam intensity b1-b7 with the apertures a1-a7 of the multi-apertureplate.

Radial displacement can be corrected by adjusting the bias voltageapplied to the Wehnelt electrode 5-8. That is, thehigh-beam-intensity-regions b1-b7 can be moved radially outward byincreasing the negative voltage applied to the Wehnelt 5-8; or radiallyinward by decreasing this negative voltage. The above adjustmentprocedure applies for the example shown in the drawings, in which theelectron gun 1-8 forms the crossover 9-8. When the electron gun does notform a crossover, however, and instead forms a single diverging beam,reducing the negative bias voltage applied to the Wehnelt 5-8 will movethe high-intensity-beam-regions radially outward, while increasing thisbias moves them radially inward. In this manner it is possible to makeadjustments to radially align the high-beam-intensity-regions b1-b7 ofthe beams incident to the multi-aperture plate 19-8 with the aperturesa1-a7 of the multi-aperture plate.

Adjustment of the alignment in azimuth can be done in two differentways: One way is to use a rotation lens 35-8 provided between theelectron gun 1-8 and the multi-aperture plate 19-8 to perform rotationabout the optical axis of the multiple beams incident to themulti-aperture plate 19-8. The other way is to provide a rotationmechanism 37-8 on the multi-aperture plate 19-8 for rotating themulti-aperture plate 19-8 about the optical axis. In this manner it ispossible to make adjustments to align, azimuth-wise, thehigh-beam-intensity-regions b1-b7 of the beams incident to themulti-aperture plate 19-8 with the apertures a1-a7 of the multi-apertureplate.

Also, by providing a rotation lens 39-8 between the multi-aperture plate19-8 and the sample 29-8, and using it to adjust the multibeam beamletsemerging from the multi-aperture plate 19-8 in rotation about theoptical axis, the orientation of the beamlet array on the surface of thesample can be accurately aligned with one of the coordinate axes (e.g.the x coordinates) of the sample, thus enabling more efficient scanningof the surface of the sample.

Also, provided below the demagnification lens 23-8 are deflectors forscanning the surface of the sample. Alignment of the directions ofdeflection of these deflectors with a coordinate axis of the sample isperformed separately. If the rotation lens 39-8 were to be providedbelow these deflectors, this would shift their directions of deflection;so this rotation lens should be provided above the demagnification lens23-8.

Also, the example shown in the drawings uses a rotation lens 35-8provided between the electron gun 1-8 and the multi-aperture plate 19-8;a rotation lens 39-8 provided between the multi-aperture plate 19-8 andthe sample 29-8; and in addition, a rotation mechanism 37-8 for rotatingthe multi-aperture plate 19-8 about the optical axis. However, when itcomes to adjusting the states of the high-beam-intensity-regions b1-b7of the beams incident to the multi-aperture plate 19-8, the positions ofthe apertures a1-a7 of the multi-aperture plate, and the orientation ofthe beamlet array on the surface of the sample, where appropriate, thefunctions of the rotation lenses 35-8 and 35-9 and the rotationmechanism 37-8 may be combined.

From the above, the following operational effects can be obtained:

-   (1) The radial positions of the high intensity beams generated by    the electron gun can be aligned with the radial positions of the    apertures of the multi-aperture plate, to thus obtain high intensity    multi-beam beamlets.-   (2) The azimuth positions of the high-beam-intensity regions in    rotation about the optical axis can be aligned with the azimuth    positions of the apertures of the multi-aperture plate, to thus    obtain high intensity multi-beam beamlets.-   (3) The orientation of the multibeam beamlet array can be accurately    aligned with the coordinate axes of the surface of the sample, to    thus enable more accurate evaluations to be performed.    Misalignment between the Secondary Electron Beam Image and a    Standard Image

In the inspection apparatus considered up to this point, the possibilityexisted for positional displacement to arise between a secondaryelectron beam image acquired by irradiating, with a primary electronbeam, a region to be inspected on a sample, and a standard imageprepared in advance, thus degrading the defect detection accuracy of theapparatus.

This positional displacement between the detected and standard imagesbecame an especially large problem when the scan region of the primaryelectron beam was sufficiently shifted with respect to the wafer tocause a portion of the inspected pattern to fall outside of the detectedimages of secondary electron beams. Moreover, this was a problem thatcould not be solved by technology that simply optimized the matchingregion within the detected image. This could be a fatal weakness,especially for inspection of high precision patterns.

To address the above problem, the present invention is configured toprevent degradation of defect detection accuracy due to positionaldisplacement between the detected image and the standard image. Anembodiment of this aspect of the invention is described below.

FIG. 42 shows a schematic representation of the configuration of thedefect inspection apparatus of the present invention. This defectinspection apparatus comprises

-   -   an electron gun 2-10, for emitting primary electron beam;    -   an electrostatic lens 8-10, for converging and shaping the        emitted primary electron beam;    -   a multi-aperture plate 12-10;    -   an ExB deflector 24-10, for directing the shaped primary        electron beam substantially perpendicularly incident to a        semiconductor wafer W;    -   an objective lens 18-10, for forming a primary electron beam        image at the surface of the wafer W;    -   a stage 60-10, provided in a sample chamber (not shown) capable        of being evacuated to a vacuum state, said stage being        configured to be movable within a horizontal plane with a wafer        W loaded thereon;    -   electrostatic lenses (28-10 and 30-10) for guiding a secondary        electron beam emitted from the surface of the wafer W to a        secondary electron detector 36-10;    -   a detector 36-10 for detecting the guided secondary electrons;        and    -   a controller 42-10 for controlling the entire apparatus, as well        as an image forming circuit 40-6 for forming a detected image        based on a secondary electron signal detected by the detector        36-10, and executing a process for detecting defects on the        wafer W based on that image.

The detector 36-10 receives the secondary electrons collected by theelectrostatic lenses 28-10 and 30-10 and coverts them to electricalsignals. As shown in detail in FIG. 47, for example, the detector 36-10has the same number of detector elements as there are multibeambeamlets, and outputs the same number of secondary electron signals inparallel to the image forming circuit 40-10. The two-dimensional imagesformed by the image forming circuit 40-10 are transferred to thecontroller 42-10.

The controller 42-10 may be a general-use personal computer (orworkstation), as shown in FIG. 42 (or may be implemented in dedicatedhardware as part of the defect detection apparatus). The controller42-10, as shown in FIG. 42, comprises

-   -   a main control unit 52-10 for executing various control and        computation processes in accordance with a prescribed program;    -   a monitor 48-10 for displaying the results of processes executed        by the main control unit 52-10; and    -   an input device 50-10 (such as a keyboard or mouse) for        inputting commands entered by an operator.

The main control unit 52-10 comprises various circuit boards not shownin the drawing (CPU RAM, ROM, hard disk, video board, etc.). Withincomputer memory (RAM or hard disk), a secondary electron image memoryarea 54-6 is allocated for storage of digital image data (secondaryelectron images of a wafer W formed from electrical signals receivedfrom the detector 36-10).

Included on the hard disk is a reference standard image memory 56-10,for storage, in advance, of reference standard images of wafers (waferswith no defects). Also stored on the hard disk, in addition to a controlprogram for controlling the overall defect inspection apparatus, is adefect detection program 58-6 for reading secondary electron image datafrom the memory area 54-6, and automatically detecting defects on awafer W, based on that image data, in accordance with prescribedalgorithms. This defect detection program 58-6 (later to be described indetail) has functions for performing a matching process to compare theactual detected secondary electron beam image with a reference standardimage read from the reference standard image memory 56-10; forautomatically detecting defects; and for displaying a warning to alertan operator when defects are detected. At this time, a secondaryelectron image 46-10 may also be displayed on the monitor 48-10.

Next, the an example of the operation of the above defect inspectionapparatus will be described with reference to the flow charts of FIGS.44 -46. First, in the main routine flow, as shown in FIG. 44, the sample(wafer W) is loaded on the stage 60-10 (Step 300-9). This loading may beaccomplished in a mode wherein a loader (not shown) capable of holding alarge number of wafers W, automatically loads wafers on the stage 60-10,one wafer at a time.

Next, images are acquired (Step 304-9) for each of multiple inspectionregions that are partially overlapping but offset position-wise withrespect to each other in the XY plane of the wafer W. These multipleinspection regions for which images are to be acquired might be, forexample, rectangular regions on the wafer inspection surface 34-9 suchas those indicated by the reference numbers 32 a-9, 32 b-9, . . . 32k-9, etc., in FIG. 48, where the different regions are displaced fromeach other position-wise around a wafer inspection pattern 30-9 yetremain partially overlapped. For example, in this step, inspectionimages 32-9 might be acquired for 16 different inspection regions, asshown in FIG. 42. Each small rectangular grid box within each of these16 images corresponds to 1 pixel. However, a ‘block’ (an area largerthan a pixel) may also be defined as the unit. The shaded grid boxescorrespond to the image of the pattern on the wafer W. A detailed flowdiagram for this step (Step 304-9), to be discussed later, is shown inFIG. 45.

Next, image data for each of the inspection regions acquired in Step34-9 is compared with the corresponding reference standard image datastored in the secondary electron image memory area 54-6 (Step 308-9 ofFIG. 3), and a decision is made as to whether defects exist in theinspection areas of the wafer covered by the above inspection regions(Step 312-9). This step involves a byte-for-byte image data matchingprocess to be described in detail later based on the process flowdiagram of FIG. 46.

If a YES decision is made in Step 312-9, based on the results of thecomparison check made in Step 308-9 (i.e., if a decision is made that adefect does exist in the inspection areas of the wafer surface coveredby the above inspection regions) a defect warning notice is output toalert the operator (Step 318-9). This may be done, for example, bydisplaying an appropriate message on the monitor 48-10. An enlarged viewof the defective pattern (46-10) may be displayed at the same time. Thedefective wafer may also be immediately removed from the sample chamber3-9 and stored in a separate location from where defect-free wafers arestored (Step 319-9).

If a NO decision is made in Step 312-9, based on the results of thecomparison check made in Step 308-9 (i.e., if no defects exist in thecurrently inspected portion of the wafer W), a decision is made as towhether there are any inspection regions in the current sample thatstill remain to be inspected (Step 314-9). If there are such regions(YES at Step 314-9), drive is applied to the stage 60-10 to position thenext inspection region of the wafer W under the irradiation region ofthe primary electron beam (Step 316-9). Execution then returns to Step302-9, to repeat the process for the next region.

If there are no more regions left to inspect on the current wafer (NO inStep 314-9),

or if execution has proceeded past the ‘pull defective wafer’ step (Step319-9), execution then proceeds to Step 320-9. In this step, a decisionis made as to whether the current sample (the wafer W), is the last oneleft to be inspected; i.e., whether there are any un-inspected wafersleft in the loader (not shown). If the answer in Step 320-9 is NO (thisis not the last wafer), the wafer just inspected is stored in its properlocation, and a new wafer takes its place on the stage 60-10 (Step322-9). Execution then returns to Step 302-9, and the same inspectionprocess is repeated for the new wafer. If the answer in Step 320-9 isYES (this is the last wafer.), the last inspected wafer is stored in itsproper location, and the process ends.

Next the process flow for Step 304-9 will be described with reference tothe flow diagram of FIG. 45. First, the image number ‘i’ is set to aninitial value of ‘1’ (Step 330-9). The image number is an identificationnumber assigned to the image of each inspection region, in the sequencein which those regions are to be inspected. Next, the system determinesan image index position (X_(i),Y_(i)) for the region of the image numberi just set (Step 332-9). The image index position defines thecoordinates of a specific point within the inspection region (e.g., itscenter) that is to be used to demarcate the region. At the current pointin the inspection process, i=1, the image index position is designated(X₁,Y₁), which corresponds, for example, to the center position of theinspection region 32 a-9 shown in FIG. 7. Image index positions aredetermined in advance for all of the inspection image regions, andsaved, for example on the hard disk of the controller 16-9, to beread-out in Step 332-9.

Next, in Step 334-9 of FIG. 45, a deflection control function of thecontroller 42-10 applies electrical potentials to the deflection poles22-10 and 24-10 as required to properly deflect the primary electronbeam passing through the objective lens 18-10 of FIG. 42 for irradiatingthe inspection image region having the image index position (X₁,Y₁), asdetermined in Step 332-9.

Next, primary electron beams are emitted from the electron gun 2-10.These beams pass through the electrostatic lenses 4-6 and 14-10, the ExBdeflector 24-10, the objective lens 18-10, and the deflection poles22-10 and 24-10, to irradiate the surface of the loaded wafer W (Step336-9). To perform this irradiation, the primary electron beams aredeflected by the electrical fields created by the deflectors 22-10 and14-10 as required to scan the entire inspection image region demarcatedby the image index position (X₁,Y₁) of the wafer inspection surface 34-9(FIG. 48). In this example, when the image number i is equal to 1, theinspection region is 32 a-9.

Secondary electron beams are emitted from the wafer surface in theinspection region irradiated by the primary electron beams. Thesesecondary electron beams are imaged on the detector 36-10 by theelectrostatic lenses 28-10 and 30-10. The detector 36-10 detects theconverged secondary electron beams and outputs an electrical signal foreach of its detector elements. These detector output signals are thenconverted to digital image data and output by the image-forming circuit40-6(Step 338-9). The digital image data for the detected image number iis then transferred to the secondary electron image memory area 54-6(Step 340-9).

In Step 342-9, the image number i is incremented by 1, and the resultingimage number (i+1) is checked to determine whether it exceeds a givenvalue i_(max) (Step 344-9). This value i_(max) is the total number ofinspection images to be acquired, or ‘16’ in the example of FIG. 42.

If a NO decision is made in Step 344-9 (if the image number i does notexceed the set value i_(max)), execution returns to Step 332-9, and theimage index position for the incremented image number (i+1), i.e.,(X_(i+1),Y_(i+1)) is determined. This image index position is at alocation displaced from the image index position (X_(i),Y_(i)),determined in the preceding routine, by a prescribed distance in the Xand/or Y directions (ΔX_(i),ΔY_(i)). In the example of FIG. 7, the nextimage index position is shifted from the coordinates (X₁,Y₁), in the Ydirection only, to the point having the coordinates (X₂,Y_(y)), wherethe inspection region then becomes the rectangular region 32 b-9, asindicated by the dotted lines in the drawing. The value to use for(ΔX_(i),ΔY_(i), where i=1, 2, . . . , i_(max)) can best be determinedfrom empirical data on how far from the field of the detector 36-10 thepattern 30-9 of the wafer inspection surface 34-9 actually shifts, thenumber of regions to be inspected, and the surface area.

Steps 332-9 through 342-9 are repeated, in sequence, i_(max) times (asmany times as there are regions to be inspected). On the waferinspection surface 34-9, as shown in FIG. 7, the location of theseinspection regions is repeatedly shifted such that the different regionspartially overlap, until the image index position becomes (X_(k),Y_(k)),(the index position after the inspection region has been changed ktimes), where the inspection region image becomes 32 k-9.

In this manner, image data is acquired for each of the 16 inspectionregions in the example of FIG. 42, and the data are stored in the memory8-9. Thus as can be understood from FIG. 42, in this example, each ofthe images 32-9 (inspection images) of the inspection regions for whichdata has been acquired includes, either partially or entirely, the image30 a-9 of the pattern 30-9 on the wafer inspection surface 32-9.

If a YES decision is made in Step 344-9 (if the incremented image numberi does exceed the set value i_(max)), Execution returns from thissubroutine to the main routine (FIG. 44), and goes on to the comparisonstep (Step 308-9).

The image data transferred to memory in Step 340-9 is composed of ‘beta’data (secondary electron intensity data) for each pixel detected by thedetector 36-10. This data however, is subjected to a series ofcomputations during the subsequent comparison step (Step 308-9 of FIG.44), in which it is matched against the reference standard image data).This image data may be saved in different states, at different stages ofthe computation, in the memory area 58-6.

This computation process may include, for example, a normalizationprocess in which the size data and/or density data of the inspectionimage is matched against the size and/or density of the referencestandard image, processes in which discrete groups of less than aprescribed number of pixels is viewed as noise and excluded from theprocess, etc. In addition, instead of using raw beta data, data may becompressed (to the extent that will not degrade detection accuracy forinspection of high precision fine patterns), by converting it tocharacterization matrices made up of characteristics extracted from thedetected patterns.

There a number of matrices that could be used for this purpose. Oneexample would be an m×n characterization matrix arrived at by dividingeach two-dimensional inspection region made up of M×N pixels into m×nblocks (m<M, n<N), and using, as matrix elements, values equal to thesums of the secondary electron intensities of the pixels contained ineach block (or a normalized value arrived at by dividing each such value(i.e., the total electron intensity of each block) by the number ofpixels in the entire region to be inspected). If this is done, thereference standard image data would be stored in the same form. Imagedata, as referred to in the present embodiment of the invention,includes, of course, raw beta data, in addition to image data in formatssuch as the above, wherein data characteristics are extracted accordingto an appropriate algorithm.

Next, the process flow for Step 308-9 will be discussed with referenceto the flow diagram of FIG. 46. First, the CPU of the controller 42-10reads reference standard image data out of the reference standard imagememory section 54-6 (FIG. 42), and transfers it to working memory inRAM, etc. (Step 350-9). In FIG. 42, this image data is identified by thereference number 46-10. The image number is then reset to i=1 (Step352-9), and the inspection image data for image number i is read fromthe memory section 8-9, and transfers it to working memory (Step 354-9).

Next, the reference standard image data and the image i data just readfrom memory are matched against each other, and the distance betweenthem is computed as a distance value D_(i) (Step 356-9). The distancevalue D_(i) is indicative of the degree of similarity between thereference standard image and the inspected image i. (The larger thevalue of D_(i), the greater the difference between the inspected imageand the reference standard image.) The actual value of D_(i) isarbitrary, as long as its magnitude expresses the degree of similarity.For example, when the image data is in the form of an M×N pixel matrix,the secondary electron intensity of each pixel (or characteristicmagnitude) may be viewed as position vector components of an(M×N)-dimensional space, and either the Euclidean distances between thereference standard image vectors and the image i vectors within this(M×N)-dimensional space, or their correlation coefficients, computed. Ofcourse, distance other than Euclidean (e.g., so-called ‘urban districtdistance’) may also be computed. In addition, if the number of pixels islarge, the computed magnitude can get quite large, and in this case thedistance between the two sets of image data (inspection and referenceimage), expressed in the form of the above m×n characteristic vectorsmay be computed instead.

I reversed the ‘greater than/less than’ and ‘yes/no’ parts of thefollowing text to make it agree with the drawing in Step 358-9 of FIG.46. This appears in the paragraph after this one.

Next, a decision is made as to whether the computed distance D_(i)exceeds a threshold value Th (Step 358-9). The value of the threshold Th(determined through experimentation) sets the standard as to whetherthere is a sufficient match between the inspected and reference standardimages. If the distance D_(i) value does not exceed the prescribedthreshold Th, (a NO decision in Step 358-9), the present inspectionsurface 34-9 of the present wafer gets a ‘no defect’ decision (Step360-9), and execution returns to this subroutine. In other words, ifeven one of the inspection images is substantially a match for thereference standard image, a ‘no defect’ decision is rendered. Since notall of the inspection images need to be subjected to this matchingprocess, this can be a high-speed decision process. In FIG. 42, thecolumn 3, row 3 inspection image, for example, has no positional offsetfrom, and is substantially matched with, the reference standard image.

If the distance D_(i) value exceeds the threshold (YES at Step 358-9),the image number i is incremented by 1 (Step 362-9), and a decision ismade as to whether the incremented image number (i+1) exceeds the setvalue i_(max) (Step 364-9). If not (if the decision in Step 364-9 isNO), execution returns to Step 354-9 to read image data for image numberi+1, and repeat the matching process for that image data. If theincremented image number i exceeds i_(max) (YES at Step 364-9), a ‘havedefect’ decision is made for the present inspection surface 34-9 of thepresent wafer W at Step 366-9, and execution returns to the mainroutine. In other words, a ‘have defect’ decision is made if there is asubstantial mismatch between the reference standard image and all of theinspection images.

In the embodiment described above, the image data-to-image data matchingprocess was performed either between pixels or between characteristicvectors, but it could be performed for a combination of the two. Forexample, high-speed matching of low-computed-magnitude characteristicvectors could be performed first; followed by a detailed pixel-to-pixelmatching process for only those inspection images determined to have ahigh degree of similarity in the first check. Such a two-stage procedurecould provide both high speed and high accuracy.

Also, in the present invention, positional displacement of theinspection image is correlated with positional displacement of theregion irradiated by the primary electron beams. The present inventioncould, however, be used in conjunction with a process performed eitherbefore or between matching processes to search for matching regionshaving the best image data match (e.g., a process for detecting regionshaving the highest coefficient of correlation therebetween, andperforming matching on those regions). Thus correlation between largedisplacement of inspection images with displacement of the regionirradiated by the primary electron beam according to the presentinvention could be determined, and regions of comparatively smalldisplacement left to be absorbed by subsequent digital image processing.Neither is the process flow limited to that shown in FIG. 44. Forexample, in the flow of FIG. 44, if a sample is judged defective at Step312-9, no further inspection for defects in other regions is performed.This flow could be changed, however, so that all regions would becovered. Also, if the region irradiated by primary electron beams can beenlarged to where substantially all of the inspection regions can becovered in one shot, Steps 314-9 and 316-9 can be eliminated.

Thus in the above process, images are acquired for multiple inspectionregions on the sample that partially overlap but are offset with respectto each other, and the images of these inspection regions are comparedwith a reference standard image, to detect defects in the sample. Thisprovides a major advantage in that it eliminates degradation of defectdetection accuracy due to positional displacement between the referencestandard image and the image being inspected.

Correcting Deviation of the Irradiated Point from the Design Point

Another improvement incorporated in the above inspection apparatus ofFIG. 42 is described below. That is, in such an inspection apparatus,when there are deviations from the design values specified for theattitude (state of rotation), positions, or separation between the aboveprimary beamlets being directed toward the stage, it is no longerpossible to perform proper inspections with the system. Therefore, thepresent system was designed so that when such deviations from designvalues occur, it will be possible to calibrate and correct for them.

At the expense of being repetitious, to best serve the purposes of thisdescription, we will start with an overview of the system shown in FIG.42. An electron beam emitted from an electron gun 2-10 is converged by acondenser lens 4-10 to form a crossover at a point 6-10. Placed belowthe condenser lens 4-10 is a first multi-aperture plate 8-10 having aplurality of apertures through which a plurality of primary electronbeamlets 12-10 are formed about an optical axis 10-10. A demagnificationlens 14-10 demagnifies these primary electron beamlets formed by thefirst multi-aperture plate 8-10, and projects them onto a plane at apoint 16-10. After being thus focused at the point 16-10, the beamletsare again focused on a sample 20-10 by an objective lens 18-10. Themultiple primary electron beamlets 12-10 emerging from the firstmulti-aperture plate 8-10 are deflected by a deflector 22-10 placedbetween the demagnification lens 14-10 and the objective lens 18-10 as ascanning device, such that they simultaneously scan the surface of thesample 20-10. Although the scanning of the multiple primary electronbeamlets 12-10 could be performed entirely by the deflector 22-10, thedeflector 22-10 and an ExB separator (to be described later) may also beused for this purpose.

As shown in FIG. 49, to eliminate the effects of image plane curvatureaberration of the demagnification lens 14-10 and the objective lens18-10, the multi-aperture plate 8-10 has provided therein, nine smallapertures 8 a-10 through 8 i-10 arranged along a circle on an endsurface thereof so that images projected toward the X-direction thereofwill be equally spaced. As shown in FIG. 50, by passing through theapertures 8 a-10 through 8 i-10 of the multi-aperture plate 8-10, theprimary electron beam 12-10 is formed into nine beamlets 12 a-10 through12 i-10, which are thus arranged to pass through a circle in positionscorresponding to those of the apertures. Also, although in the presentexample, the apertures are arranged along a circle, this is not arequirement: they could instead be arranged in a straight line. Also,although nine apertures were used, there as few as two could be used.

Multiple points on the sample 20-10 are irradiated by the convergedprimary electron beamlets 12-10. The secondary electron beamlets thatare then emitted from these irradiated points are attracted and tightlyconverged by the electric field of the objective lens 18-10 anddeflected by the ExB separator 24-10, to thus separate them from theprimary electron beamlets irradiating the sample 20-10, and inject theminto the secondary optical system.

The reference number 17-10 in FIG. 42 identifies an axial alignmentdeflector, and the reference number 19-10 identifies an axiallysymmetrical electrode. Also, a rotation lens 23-10 that is capable ofrotating the multiple primary electron beamlets 12-10 is providedbetween the multi-aperture plate 8-10 that forms part of an electronbeam forming device, and the ExB separator 24-10 that functions as aseparation device. More specifically, in this mode of the invention, therotation lens 23-10 is placed near the point 6-10 of FIG. 42. Thisrotation lens 23-10 is capable of rotating the primary electron beamlets12-10 about the optical axis, responsive to the intensities of theexcitation currents flowing in its coils.

The secondary optical system has magnification lenses 28-10 and 30-10for imaging the secondary electron beams passed therethrough at themultiple apertures 34 a-10-34 i-10 of a second multi-aperture plate34-10. After passing through multiple apertures, the secondary electronbeams are detected by the multiple detector elements 36 a-10-36 i-10.(The second multi-aperture plate 34-10 is placed in front of thedetector elements 36 a-10 through 36 i-10.) The apertures 34 a-10-34i-10 are formed in the second multi-aperture plate 34-10 along a circleon an end surface thereof such as to correspond on a one-for-one basiswith respective apertures 8 a-10-8 i-10 formed in the firstmulti-aperture plate 8-10. To show this correspondence in FIG. 49, theapertures 34 a-10-34 i-10 of the second multi-aperture plate 34-10 areindicated by dotted lines. The detector elements 36 a-10-36 i-10 areplaced opposite the apertures 34 a-10-34 i-10, respectively, of thesecond multi-aperture plate 34-10, and are therefore placed along acircle as the apertures. (For convenience, the detector elements 36a-10-36 i-10 are shown schematically as blocks in FIG. 42.) Each of thedetector elements 36 a-10-36 i-10 converts its secondary electronbeamlet to an electrical signal representative of the intensity of thatbeamlet. Each of these electrical signals output by the detectorelements is then amplified by an amplifier 38-10, and input to the imageprocessor 40-10, where it is converted to image data. A signal that isthe same as the scan signal applied to the deflector 22-10 fordeflecting the primary electron beams is also supplied to the imageprocessor 40-10 by the controller 42-10. Using position data from thescan signal, and secondary electron density data from the secondaryelectron signals, the image processor 40-10 generates image data fromwhich it can construct and display an image of the scanned surface ofthe sample 20-10.

The image processor 40-10 is connected to the controller 42-10 forconducting data communication therebetween. The controller 42-10, may bea general use personal computer, as shown in FIG. 42. This computercomprises a main control unit 44-10 for executing control andcomputation processes in accordance with a prescribed program; a monitor48-10 for displaying the results of these processes, and secondaryelectron images 46-10; and an input device 50-10 (such as a keyboard ormouse) for inputting commands entered by an operator. The controller42-10 may also, of course, be configured as dedicated defect inspectionsystem hardware, or as a workstation, etc.

The main control unit 42-10 comprises various circuit boards and majorcomponents not shown on the drawing (CPU RAM, ROM, hard disk, videoboard, etc.). A memory unit 52-10 is connected to the controller 44-10.This memory unit 52-10 could be, for example, a hard disk. Within thismemory unit 52-10, is a secondary electron image memory area 54-10,allocated for storage of sample 20-10 secondary electron image datareceived from the image processor 40-10; and a reference standard imagememory area 56-10, allocated for advance storage of reference standardimage data taken from defect-free samples. Also stored in the memoryunit 52-10 is a control program for controlling the overall electronbeam apparatus; a sample evaluation program; and a control program 58-6,for performing calibrations and making corrections, in order to correctfor deviations from the design values specified for the attitude (stateof rotation), positions, or separation between multiple primary beamletsbeing directed toward the sample, when such deviations occur. Thecontrol method for calibrating primary electron beam deviations of thistype will be explained in detail later, but it should be noted at thispoint that calibration is performed before performing any sampleevaluations, in order to establish the initial primary electron beamsettings.

The image data for the scanned surface of the sample 20-10 (stored inthe secondary electron image area memory 54-10) is compared againststandard reference image data taken from defect-free samples (stored inadvance in the reference standard image memory 56-10) in order to detectthe presence of defects in the sample 20-10.

The line width of a pattern on a sample 20-10 can be measured asfollows: The sample 20-10 pattern to be evaluated is moved into theproximity of the optical axis 10-10 of the primary optical system, and aline scan is performed to obtain a line width evaluation signal. Thatsignal is then calibrated by a procedure (registration calibration) tobe described later.

To measure the X position of the stage 60, a moving mirror is providedby mounting a laser-reflecting mirror 62-10 on an X-direction end of thestage 60-10; and a stationary mirror is provided by mounting a mirror onthe objective lens. The distance between the fixed and moving mirrors isconstantly measured by establishing interference between a laser beamemitted by a laser 64-10 and reflected from the stationary mirror, and abeam reflected from both the moving mirror and the stationary mirror.Measurement signals obtained from the resulting interference patternsare sent to the controller 42-10, which uses the signals to compute theX position of the stage. Also, a moving mirror is mounted on aY-direction end of the stage 60-10 (not shown), and a fixed Y-directionmeasurement mirror is mounted on the end of the objective lens, suchthat the Y position of the stage 60-10 can be measured from theinterference resulting when a laser beam emitted by a laser (not shown)positioned outward of the stage, is reflected by the two mirrors.

A mark pad 66-10 is provided on one side of the stage 60-10. The surfaceof the mark pad 66-10 constitutes an XY coordinates plane (FIG. 50). Asshown in FIG. 50, provided on the mark pad 66-10, are two beam positionmeasurement marks, 66 a-10 and 66 e-10. The marks 66 a-10 and 66 e-10are provided along the X-axis, substantially parallel thereto. (Theirdegree of parallelism to the X-axis is measured beforehand and stored inthe memory unit 52-10 of the controller 42-10 as a system constant.) InFIG. 50, the center point between the mark 66 a-10 and 66 e-10 isindicated by the letter O.

Using the stage drive system described above, the stage 60-10 can bemoved in the X and Y directions to a position at which the surface ofthe mark pad 66-10 can be irradiated by the primary electron beams12-10. When this is done, a plurality of irradiation points 12 a-10-12i-10 are formed around a circle on the surface of the mark pad 66-10.(That is, the primary electron beamlets 12 a-10-12 i-10 are formed atthe surface of the mark pad 66-10, as shown in FIG. 50.) The marks 66a-10 and 66 e-10 are formed so that the distance between them will beless than the scan width used when the marks are scanned. This is doneso that one primary electron beamlet will not be able to irradiate bothmarks in the same scan. Because there is exact correspondence betweenthe primary beamlets and the detector elements this will also preventthe signal produced by the scanning of the mark by one primary beamletfrom being mistaken for the signal produced by another.

Next, the electron beam calibration method will be described withreference to FIGS. 51 through 53( c). First, the stage drive system isoperated to drive the stage 60-10 as required to move the mark pad 66-10under the optical axis, and align the position of the optical axis 10-10with the center point O between the marks 66 a-10 and 66 e-10 (Step68-10 of FIG. 51). This alignment should be accurate to within aprescribed error margin. The above can be done, for example, by scanningthe primary electron beams over a prescribed range in the X and Ydirections, and confirming that the marks 66 a-10 and 66 e-10 aredetected by the primary beamlets 12 a-10 and 12 e-10 (the beams that areon the X axis) that form the irradiation points 12 a-10 and 12 e-10.When alignment is performed in this manner between the optical axis10-10 and the mark pad 66-10, primary electron beamlets 12 a-10 and 12e-10 irradiate the pad in the vicinity of the marks 66 a-10 and 66 e-10as shown, for example, in FIG. 52( a). This enables the primary electronbeamlets 12 a-10 and 12 e-10 to be scanned so as to cut across the marks66 a-10 and 66 e-10 by scanning the beamlets over their prescribedranges in the X and Y directions.

In the example shown in FIG. 52( a), the distance between the mark 66a-10 and the irradiation point of the primary electron beamlet 12 a-10is longer in both the X and Y directions than the distance from the mark66 e-10 to the irradiation point of the primary electron beamlet 12e-10. That is, in this example, the distance between the primaryelectron beamlet 12 a-10 and 12 e-10 irradiation points and the marks 66a-10 and 66 b-10 represents the deviation of the primary electronbeamlets from their predicted values (i.e., the multibeam deviation).

When alignment of the optical axis 10-10 and the mark pad 66-10 isperformed, the position of the stage 60-10 is computed (using thelaser-reflecting mirror 62-10 and the linear measurement laser 64-10)and these position data are then stored in the memory 52-10 as the markposition system constant. Normally, this alignment of the optical axis10-10 and the mark pad 66-10 positions needs to be performed only once,when the electron beam apparatus initial settings are performed. Themark position system constant, based on the above alignment, however, isupdated based on a calibration process to be described later.

Next, the positioning of the primary electron beamlets relative to eachother is measured. In the present embodiment, this measurement isperformed by providing two marks on a mark pad, and scanning twoelectron beamlets 12 a-10 and 12 e-10, which form irradiation points inthe vicinity of the two marks 66 a-10 and 66 e-10, so that the beamswill cut across the marks. To illustrate this embodiment as clearly aspossible, the other electron beamlets (b-d and f-i) are not shown inFIGS. 52 through 53( c).

First, under direction of the control program 58-10 (stored in thememory unit), the controller 42-10 controls the deflector 22-10 to scanthe electron beamlets 12 a-10-12 e-10 in the X-direction, toward themarks 66 a-10 and 66 e-10, as shown in FIG. 52( a). When this is done,the primary electron beamlet 12 e-10 crosses over the mark 66 e-10first, after which, the primary electron beamlet 12 a-10 crosses overthe mark 66 a-10. Therefore, as shown in FIG. 53( a), the signal 90-10output by the detector element 36 e-10 (the signal corresponding to theprimary electron beamlet 12 e-10) appears first, followed by the signal92-10, which is output by the detector element 36 a-10 (the signalcorresponding to the primary electron beamlet 12 a-10). FIG. 53( a)shows signal strength on the vertical axis and time on the horizontalaxis. Since the primary electron beam scan rate (μm/μsec) in the X-axisdirection is known in advance, the X-direction distance between the mark66 a-10 and the primary electron beamlet 12 a-10 (i.e., the irradiationpoint 12 a-10), and the X-direction distance between the mark 66 e-10and the primary electron beamlet 12 e-10 (i.e., the irradiation point 12e-10), can be computed from the times at which the two signals 90-10 and92-10 are output. The time between the two signals 90-10 and 92-10 can,of course, also be used to compute the distance between the two primaryelectron beamlets 12 a-10 and 12 e-10 (i.e., their irradiation points 12a-10 and 12 e-10).

Next, the time difference between the two signals 90-10 and 92-10 iscomputed, and a decision made as to whether this time difference fallswithin the error tolerance for its design value (Step 72-10 of FIG. 51,which uses a measurement device.) If the time difference is not withintolerance, a zoom operation is performed with the intermediate lens14-10 and the objective lens 18-10 of FIG. 42 under control of thecontroller 42-10. That is, the demagnification ratio is changed withoutchanging the object position 8-10 and the image position 20-10. (Step74-10 of FIG. 51, which uses a two-stage lens control device.) Thischanges the X position of the primary electron beamlet 12 a-10 (i.e. theirradiation point 12 a-10) with respect to the mark 66 a-10, and the Xposition of the primary electron beamlet 12 e-10 (i.e. the irradiationpoint 12 e-10) with respect to the mark 66 e-10. In this manner, steps70-10, 72-10 and 74-10 are repeated until the waveforms of the signals90-10 and 92-10 appear at substantially the same point in time. Whenthis is done, on the mark pad 66-10, the distance in the X directionbetween the mark 66 a-10 and the primary electron beam 12 a-10 (i.e. theirradiation point 12 a-10), and the distance in the X direction betweenthe mark 66 e-10 and the primary electron beam 12 e-10 (i.e. theirradiation point 12 e-10) will be substantially equal, as shown in FIG.52( b). When a decision is made that the time difference between the twosignals 90-10 and 92-10 is within tolerance (Step 72-10), the values ofthe excitation voltages applied to the intermediate lens 14-10 and theobjective lens 18-10 at this time are stored in the memory unit 52-10(Step 73-10).

Next, under direction of the control program 58-10, stored in the memoryunit 52-10, the controller 42-10 controls the deflector 22-10 to scanthe electron beamlets 12 a-10-12 e-10 in the Y-direction, toward themarks 66 a-10 and 66 e-10, as shown in FIG. 52( b) (Step 76 of FIG. 51).When this is done, the primary electron beamlet 12 e-10 crosses over themark 66 e-10 first, after which, the primary electron beamlet 12 a-10crosses over the mark 66 a-10. Therefore, as shown in FIG. 53( b), thesignal 94-10 output by the detector element 36 e-10 (the signalcorresponding to the primary electron beamlet 12 e-10) appears first,followed by the signal 96-10, which is output by the detector element 36a-10 (the signal corresponding to the beamlet 12 a-10). FIG. 53( b)shows signal strength on the vertical axis and time on the horizontalaxis. Since the primary electron beam scan rate (μm/μsec) in the Y-axisdirection is also known in advance, the Y-direction distance between themark 66 a-10 and the primary electron beamlet 12 a-10 (i.e., theirradiation point 12 a-10), and the Y-direction distance between themark 66 e-10 and the primary electron beamlet 12 e-10 (i.e., theirradiation point 12 e-10), can be computed from the times at which thetwo signals 94-10 and 96-10 are output.

Next, the time difference between the two signals 94-10 and 96-10 iscomputed, and a decision made as to whether this time difference fallswithin the error tolerance for its design value (Step 78-10 of FIG. 51).If the time difference is not within tolerance, and the signal 94-10occurs first in time, as shown in FIG. 53( b), this shows that theprimary electron beamlets are rotated counterclockwise, with the centerpoint 0 as the center of rotation, as shown by the arrow in FIG. 52( b).When this kind of misalignment exists, the controller 42-10 adjusts theintensity of the excitation current supplied to the rotation lens 23-10shown in FIG. 42 (Step 80-10 of FIG. 51), thus rotating the primaryelectron beamlets clockwise. Steps 76-10, 78-10, and 80-10 are repeatedas required to adjust the signals 94-10 and 96-10 to where they bothappear as close as possible (within tolerance) to the same point intime. When this is done, on the mark pad 66-10, the distance in the Ydirection between the mark 66 a-10 and the primary electron beam 12 a-10(i.e. the irradiation point 12 a-10), and the distance in the Ydirection between the mark 66 e-10 and the primary electron beam 12 e-10(i.e. the irradiation point 12 e-10) will be substantially equal. Inthis state, a line drawn between the marks 66 a-10 and 66-10 will besubstantially parallel to a line drawn between the primary electron beam12 a-10 (the irradiation point 12 a-10) and the primary electron beam 12e-10 (the irradiation point 12 e-10). In other words, in this state,there is no rotation error. When a decision is made that the timedifference between the two signals 94-10 and 96-10 is within tolerance(Step 78-10), the value of the rotation lens 23-10 excitation current atthis time is stored in the memory unit 52-10 (Step 79-10).

When the above calibration process from step 68-10 through 80-10 hasbeen performed, on the mark pad 66-10, the distance xa, the distance inthe X direction between the mark 66 a-10 and the primary electron beam12 a-10 (i.e. the irradiation point 12 a-10), and the distance xe, thedistance in the X direction between the mark 66 e-10 and the primaryelectron beam 12 e-10 (i.e. the irradiation point 12 e-10) will besubstantially equal, as shown in FIG. 52( c). Also on the mark pad66-10, the distance ya, the distance in the Y direction between the mark66 a-10 and the primary electron beam 12 a-10 (i.e. the irradiationpoint 12 a-10), and the distance ye, the distance in the Y directionbetween the mark 66 e-10 and the primary electron beam 12 e-10 (i.e. theirradiation point 12 e-10) will be substantially equal.

Finally, the amount of alignment between the positions of the opticalaxis and the marks is either computed or measured, as described below.After the above calibration process has been performed, the signals90-10 and 92-10 are output at substantially the same time when the beamsare scanned in the X direction and the signals 94-10 and 96-10 areoutput at substantially the same time when the beams in the Y direction.Therefore, this means that, as shown in FIG. 53( c), the deflectionvoltage 92-10 (the deflector 22-10 deflection voltage) that exists whenthe signal 90-10 is output, is equal to the deflector 22-10 deflectionvoltage that exists when the signal 92-10 is output. Also, the deflector22-10 deflection voltage that exists when the signal 94-10 is output, isequal to the deflector 22-10 deflection voltage that exists when thesignal 92-10 is output. Also, as described above, the values of theexcitation voltages applied to the lenses 14-10 and 18-10 when thesignals 90-10 and 92-10 are output at substantially the same time, andthe excitation current supplied to the rotation lens 23-10 when thesignals 94-10 and 96-10 are output at substantially the same time arestored in memory (Steps 73-10 and 79-10 of FIG. 51). The deflectionsensitivity (μm/mv) of the deflector 22-10 is also stored in the memory52-10.

Therefore, the number of microns by which the positions of the primaryelectron beamlets are displaced from the positions of the marks in the Xdirection, when the beams are scanned in the X direction (i.e., thedistances xa and xe in FIG. 52( c)), is computed based on the deflectorsensitivity (μm/mv), which is known in advance, and the deflectionvoltage that exists when the beams are scanned in the X direction. Also,the number of microns of Y-direction positional displacement between thebeams and the marks, when the beams are scanned in the Y direction(i.e., the distances ya and ye) are computed, based on the abovedeflection voltages (Step 84-10 of FIG. 51). Then, the mark positionsystem constants are updated to new values by adding these positionaldisplacement values to stage position data previously measured byinterferometer, and stored in memory (Step 86-10 of FIG. 51).

The measurements shown in FIGS. 52( a) and (b) and FIGS. 53( a) and (b)are immune to stage vibration measurement error. With respect to themeasurements shown in FIG. 52( c) and FIG. 53( c) as well, sincedimensions computed from the deflector sensitivity are added to laserlength measurement device readings taken at the instant of measurement,these measurements are also immune to stage vibration error. Thus themeasurements shown in FIGS. 52( c) and 53(c) may be taken while thestage is in motion.

According to the above embodiment, evaluation of semiconductor wafershaving minimum pattern line widths of 0.1 μm or less can be performedwith high throughput and reliability. Also, measurement of the rotationof, and separation between, the multiple electron beams (multibeambeamlets), and of the positional displacement between beams and markscan be performed without the measurements being affected by stagevibration. In addition, because the primary electron beams can beprecisely aligned with the XY coordinates of a mark pad, images can beformed without performing complex computations.

The above calibration method can be performed automatically by a controlsystem, under control of a program stored in memory; or it can beperformed manually while observing image data on a monitor.

Also, in the present embodiment, the calibration process was describedusing two of the nine primary electron beamlets in the system, but itshould be noted that at least two primary electron beams are required toperform the calibration.

In the present invention as described above, the positionaldisplacements of irradiation points relative to position measurementmarks are measured. When positional displacement is measured, theirradiation points of the multiple primary electron beamlets are thencalibrated (and related system constants are updated) based on thatpositional displacement. Therefore, even when deviations from designvalues develop in terms of the position and attitude (rotation) of themultiple electron beamlets incident to the stage, and the distancebetween those electron beams, since the system can be calibrated tocorrect for these deviations, it will still be possible to perform highresolution, high throughput inspections for defects in samples.

Correcting Aberrations (Adjusting Crossover Position)

In the prior electron beam inspection apparatus described earlier, anaperture was provided for adjusting the position of a crossover formednear the objective lens. To adjust the crossover position, the beamdiameter was measured as this aperture was being shifted along theoptical axis. The point at which the smallest beam diameter was measuredwas then used as the crossover position. The problem with thisadjustment method, however, was that the characteristics of the lensesadjacent to the aperture were dramatically changed when the aperture wasmoved, and the design characteristics of the lenses could therefore nolonger be realized.

On the other hand, since all or a portion of the multiple electron beamsare positioned away from the optical axis, the crossover positioninfluences a number of aberrations, including image distortion,magnification chromatic aberration, rotation chromatic aberration, fieldplane curvature aberration, and field astigmatism, with the influence ofmagnification chromatic aberration and rotation chromatic aberrationbeing especially significant.

It is therefore also an object of the present invention to provide thecapability to adjust the position of the crossover near the objectivelens such that aberrations can be corrected without affecting the lenscharacteristics. The crossover position adjustment method of the presentinvention is described below.

FIG. 54 shows a schematic representation of an electron optical systemfor describing the adjustment. It is essentially the same as the otherelectron optical systems described up to this point. In this opticalsystem, multiple primary electron beams 20-11 formed by a firstmulti-aperture plate 3-11 are demagnified by a demagnification lens 5-11to be projected at a point 15-11. After being focused at the point15-11, the beams are focused on a sample W by an objective lens 7-11.The multiple primary electron beams 20-11 are also converged by thedemagnification lens 5-11 to form a crossover at a point 24-11. Thiscrossover position 24-11 is near the objective lens 7-11 (or morespecifically, between the objective lens 7-11 and an ExB separator 6-11that will be discussed later). By adjusting this position in the Z-axisdirection, any one of the following aberrations can be reduced almost tozero: magnification chromatic aberration, rotation chromatic aberration,distortion, landing angle, and coma.

Also shown in FIG. 54 are an axis alignment deflector 17-11, and anaxially symmetrical electrode 18-11. Also provided between themulti-aperture plate 3-11 that forms part of the multibeambeamlet-forming system and the ExB separator 6-11, is a rotation lens22-11, for rotating the multiple electron beams 20-11. Morespecifically, the rotation lens 22-11 is located near the point 4-11.This rotation lens 22-11 is capable of rotating multiple primaryelectron beamlets 20-11 about the optical axis, responsive to theintensity of excitation currents flowing in its coils. A sample W isloaded on a stage 60-11. The stage 60-11 made so that it can be moved,by a stage drive system not shown in the drawing, in an X direction(left and right in FIG. 54), a Y direction (perpendicular to the page onwhich FIG. 54 appears), and a Z-axis direction (up and down in FIG. 54).

Also provided is a laser 31-11, for generating a laser beam. This laserbeam is split into two beams by a half-reflecting mirror 61-11. Of thesetwo beams, the one that passes through the half-reflecting mirror 16-11becomes incident to a moving mirror 40-11 that is attached to anX-direction end of the stage 60-11. The other beam from thehalf-reflecting mirror 61-11 is reflected by a fully-reflecting mirror62-11 to become incident to a stationary mirror 39-11 that is providedon the objective lens 7-11. The two beams are thus reflected from themoving mirror 40-11, and the stationary mirror 39-11, respectively. Thebeam reflected from the moving mirror 40-11 passes through thehalf-reflecting mirror 16-11 to be guided to a receiver 63-11. The beamreflected from the stationary mirror 39-11 is reflected by thefully-reflecting mirror 62-11 and the half-reflecting mirror 61-11 toalso be guided to the receiver 63-11. In the receiver 63-11,interference light of the beams reflected from the moving and stationarymirrors (40-11 and 39-11) is detected. The detected signal is sent to aCPU 32-11, which determines therefrom, the distances in the X and Ydirections between the moving and stationary mirrors (40-11 and 39-11);i.e., the x,y coordinates of the stage 60-11 position. A mark (notshown) is also provided on the stage 60-11. The stage 60-11 can be movedin the X and Y directions by the drive system mentioned above, to alocation wherein the surface of the mark can be scanned by the primaryelectron beams 20-11. The positions of the beams 20-11 can now bedetected by scanning the beams over the mark.

The CPU 32-11 is connected to the cathode power supply 25-11 for thecathode 30-11 of an electron gun 1-11 such that the voltage applied tothe cathode 30-11 can be controlled by the CPU 32-11 to varyperiodically between a few tens of volts and a few hundred volts.Changes in beam position that occur when the voltage applied to thecathode 30-11 is varied in this manner can then be detected by the CPU32-11, and used to measure the movement of the beams. In other words,the movement on the sample W of the multiple beams 20-11 in thedirection of irradiation, and in the direction of rotation centered onthe optical axis, can be measured, by varying the voltage applied to thecathode 30-11 between a few tens of volts and a few hundred volts, in aperiodic cycle. This changing of voltage corresponds to changes in theenergy of the beams, which means that when movement of the beams inrotation and in the direction of radiation is minimized, themagnification chromatic aberration and rotational chromatic aberrationwill also be minimized.

As mentioned above, a crossover is formed by the multiple primaryelectron beams 20-11 at the crossover position 24-11, near the objectivelens 7-11. The above measurement can be performed by CPU 32-11 such asto adjust the crossover position 24-11 along the optical axis to a pointat which the above movement of the primary electron beams 20-11 isminimized. This, then, measures the Z-axis position of the crossover24-11 at which the least magnification chromatic aberration or rotationchromatic aberration occurs. Preferably, this adjustment should beperformed after axial alignment of the objective lens 7-11 has beencompleted. The axial alignment of the objective lens 7-11 can beperformed by superimposing an axial alignment power supply voltage onthe deflector 21-11.

As stated above, in the past, the use of an aperture to adjust theposition of this crossover resulted in huge changes in the electronoptical system lens characteristics, causing major problems. Accordingto the embodiment of the invention just described, however, by changingthe voltage applied to the cathode 30-11, and adjusting it such as tominimize the movement, on a sample W, of the multiple primary electronbeams 20-11, as described above, to thereby adjust the position 24-11(on the optical axis) of a crossover formed near the objective lens 7-11by the multiple primary electron beams 20-11, the above adjustment canbe performed without affecting the characteristics of the lenses.

Also by adjusting the position along the optical axis of a crossover24-11 formed near the objective lens 7-11 by the multiple primaryelectron beams 20-11, in addition to landing angle corrections, any oneof the following aberrations can be corrected: image distortion,magnification chromatic aberration, rotation chromatic aberration, coma,and field astigmatism. Also, as discussed above, if an aperture platesuch as that shown in FIG. 8. is used, no problems in terms of imagedistortion or image curvature aberration will be encountered, nor willfield astigmatism or landing angle be affected. Therefore, crossoverposition can be adjusted to correct for magnification chromaticaberration and rotation chromatic aberration without causing otherproblems.

If, when this is done, an electrostatic lens is used for the objectivelens, since this lens has nothing to do with rotation chromaticaberration, the adjustments of the present embodiment can be used tocorrect for magnification chromatic distortion only. Also, ifmagnification chromatic aberration is corrected, this will make itpossible to increase the number of primary electron beams (multibeambeamlets), thus enabling higher throughput for evaluation of samplessuch as wafers and masks.

Noise Reduction

In the electron beam inspection apparatus of the present invention, inorder to perform high reliability inspections, the quantity of secondaryelectrons detected per pixel must be on the order of 4000electrons/pixel. This requires high intensity beams.

When an electron gun is operated in the temperature-limited region ofits current vs. temperature characteristic curve, there is a largeamount shot noise. It is known, however, that when the gun is operatedin the space-charge-limited region, the shot noise is reduced toapproximately 13% of that in the temperature-limited region. Therefore,to most effectively improve the signal/noise (S/N) ratio of the signal,the electron gun should be operated in the space-charge-limited region.This will reduce, by a factor of 0.13²≈0.017, the number ofelectrons/pixel that must detected to have the same S/N ratio.

An advantage of operating the electron gun in the temperature-limitedregion, however, is that in this region, the position of the electronbeam crossover can be changed arbitrarily, without changing thebrightness or emission current, by changing the Wehnelt voltage orcontrol anode voltage. This makes it easy to align the direction ofstrongest electron beam emission from the electron gun with the area ofthe aperture plate containing the multiple apertures.

On the other hand, when the electron gun is operated in thespace-charge-limited region, a change in the Wehnelt voltage or controlvoltage causes large changes in brightness and emission current, whichmakes it difficult to control the vertical position of the crossover.This is a problem because it makes it difficult to align the directionof strongest electron beam emission from the electron gun with the areaof the aperture plate that has the multiple apertures.

The present invention provides an electron beam inspection apparatus inwhich the electron gun can be operated in the space-charge-limitedregion to thereby reduce the electron beam shot noise. An embodiment ofsuch an electron beam inspection apparatus is described below.

FIG. 55 shows a typical electron beam apparatus 1-12 havingsubstantially the same configuration as those described earlier. Theapparatus comprises a primary optical system 10-12; a secondary opticalsystem 30-12; a detector unit 40-12; and a control unit 50-12. Theprimary optical system 10-12 comprises

-   -   an electron gun 11-12;    -   condenser lenses 12-12 and 13-12 for converging the electron        beams;    -   an aperture plate 14-12;    -   electrostatic alignment deflectors 15-12 and 16-12;    -   a knife-edge 17-12 for blanking;    -   a demagnification lens 18-12 for demagnifying electron beams        passed through the aperture plate 14-12;    -   an electrostatic deflector 19-12;    -   an ExB separator 20-12; and    -   an objective lens 21-12.

All of the above are placed with the electron gun 11-12 in the top-mostposition, such that an optical axis A of the electron beams emitted fromthe electron gun 11-12 will be perpendicular to the surface of a sampleS. Also, placed under the electron gun is an electrostatic axialalignment deflector 23-12, and placed between the condenser lenses 12-12and 13-12, are electrostatic alignment deflectors 24-12, and 25-12. Thecathode 111-12 of the electron gun 11-12 is constructed with a pluralityof tips, equal in number to the electron beamlets to be emitted, andarranged concentric with the optical axis A. The electron gun 11-12further comprises a Wehnelt electrode 112-12 and an anode 113-12 forcontrolling the electron gun within the space-charge-limited region byvarying, to some extent, the Wehnelt bias.

The secondary optical system 30-12 comprises two electrostaticmagnification lenses 31-12 and 32-12 placed along an optical axis B thatis inclined with respect to the optical axis A from a point near the ExBseparator 20-12; and an aperture plate 33-12 having formed therein, aplurality of apertures, arranged in a two-dimensional array.

The detector unit 40-12 comprises a detector 41-12, an amplifier 42-12,and an image processor 43-12. The control unit 50-12 comprises adeflector controller 51-12; and a computer 52-12, for controlling theimage processor 43-12 and the deflector controller 51-12.

The operation of this electron beam inspection apparatus is the same asthat of the systems described earlier. The operating point of theelectron gun 11-12 can be controlled within its space-charge-limitedregion by varying, to some extent, the Wehnelt bias.

Another electron beam inspection apparatus 1′-12 is shown in FIG. 56.Components of the apparatus in FIG. 56 that are identical tocorresponding components of the apparatus of FIG. 55 are assigned thesame reference numbers in both drawings, and elements of FIG. 56 thathave corresponding elements in FIG. 55 but are of different constructionhave the same reference numbers, but with a ‘prime’ (′) symbol added.

As in the first embodiment, the electron beam inspection apparatus 1′-12of the present embodiment comprises a primary optical system 10′-12; asecondary optical system 30-12; a detector unit 40-12; and a controlunit 50-12.

The primary optical system 10′-12 comprises an electron gun 11′-12;

-   -   condenser lenses 12-12 and 13-12 for converging the electron        beams;    -   an aperture plate 14′-12 having a plurality of apertures 141-12        therein;    -   electrostatic alignment deflectors 15-12 and 16-12;    -   a knife-edge 17-12 for blanking;    -   a demagnification lens 18-12 for demagnifying electron beams        passed through the aperture plate 14′-12;    -   an electrostatic deflector 19-12;    -   an ExB separator 20-12; and    -   an objective lens 21-12.

All of the above are placed with the electron gun 11′-12 in the top-mostposition, such that an optical axis A of the electron beams emitted fromthe electron gun will be perpendicular to the surface of a sample S.Also, as in the primary optical system 10-12 of the first embodiment,placed under the electron gun is an electrostatic axial alignmentdeflector 23-12, and placed between the condenser lenses 12-12 and13-12, are electrostatic alignment deflectors 24-12, and 25-12.

The cathode 111′-12 of the electron gun 11′-12 is constructed with aplurality of tips, equal in number to the electron beamlets to beemitted, and placed concentric with the optical axis A. This electrongun 11′-12 also comprises a Wehnelt electrode 112-12 and an anode113-12, for controlling the electron gun within the space-charge-limitedregion by varying to some extent, the Wehnelt bias.

In this embodiment, the aperture plate 14′-12 is placed under acrossover C1 that is formed under the condenser lens 12-12, by thecondenser lens 12-12, with the condenser lens 13-12 placed under theaperture plate 14′-12.

The secondary optical system 30-12, detector 40-12, and control unit50-12 all have constituent elements and placements thereof that areexactly the same as in the first embodiment.

In the above configuration, a plurality of electron beams C emitted fromthe electron gun 11′-12 form a crossover C1 near the anode 112-12, anddiverge therefrom at a fairly small angle. The diverging electron beamsare then converged by the short-focal-length condenser lens 12-12 toform a crossover C2 near the condenser lens 12-12. The aperture plate14′-12 is placed at a substantial distance from the crossover C2. Theelectron beams C diverging from crossover C2 irradiate the apertureplate 14′-12 and pass through the apertures 141-12 formed therein tobecome multibeam beamlets. These multibeam beamlets are converged by thecondenser lens 13-12 to be imaged at the crossover C3. Placed at thislocation is the blanking knife-edge 17-12. The beamlets passed throughthe multiple apertures 141-12 are demagnified by the demagnificationlens 18-12 to be projected at C4. After being focused at C4, thebeamlets proceed on toward the sample S to be imaged by thereon by theobjective lens 21-12. Commanded by the computer and deflectorcontroller, the multibeam beamlets formed by the aperture plate 14′-12are simultaneously scanned via the electrostatic scan deflector 19-12over the surface of the sample S. After this point, operation is thesame as in the apparatus of FIG. 55.

In both of the first and second embodiments described above, if thecrossover C2 is moved vertically toward the electron gun to the positionat point C5 of FIG. 56, the positions of the electron beams in thedirection in which the strongest portions of the beams are emitted fromthe electron gun 11-12, 11′-12 can be directed such that the beams willexpand outward on the aperture plate 14-12, 14′-12 (as indicated by thearrow D). Conversely, if the crossover C2 is moved vertically toward theposition of the condenser lens 13-12, the positions of the electronbeams in the direction in which the strongest portions of the beams areemitted from the electron gun 11-12, 11′-12 can be directed such thatthe beams are drawn inward on the aperture plate 14-12, 14′-12. In thismanner, the direction of strongest electron beam emission can beadjusted to align it with the area containing the multiple apertures ofthe aperture plate 14-12, 14′-12. These adjustments can easily beperformed by changing only the excitation of the condenser lens 12-12,without changing the operating conditions of the electron gun 11′-12.Therefore, the electron gun can be operated at a desired operating pointwithin the space-charge-limited region such as to dramatically decreasethe shot noise generated by the electron beam to approximately 1.8% ofthat experienced when operating in the temperature-limited region.

As described above, in the electron beam inspection apparatusillustrated in FIG. 55 and FIG. 56, (1) a dramatic reduction in shotnoise can be achieved by operating the electron gun in thespace-charge-limited region, which in turn results in lower secondaryelectron noise.

Abnormal Dose

In the above multibeam inspection apparatus, if an increase inthroughput were to be sought by simply increasing the speed at which thestage is moved, this would decrease the total amount of beam currentwith which the sample is irradiated (hereinafter, dose), thus degradingthe sample image. Thus when stage speed is increased, the beam currentof the electron beams must also be increased.

For this reason, in prior multibeam inspection systems, stages werecontinuously scanned at high speed while continuously irradiating thesample with high current beams. Even when high speed scanning wasperformed under these conditions, however, there were times when thestage would stop or slow down for one reason or another. When suchslowing or stopping of the stage occurred during high speed scanning,the high current beams would continue to irradiate the same spots orvicinities on the surface of the sample, causing an abrupt increase indose. A sample can tolerate increased dose only within a limited range,however, and if the sample continues to be irradiated after the maximumallowable radiation dose has been exceeded, the sample could becomecontaminated or charged, or in the worst case, could even be destroyed.

In consideration of this problem, the present invention provides amultibeam inspection apparatus such that when the sample is beingirradiated by electron beams as relative motion is effected between thebeams and the sample, any abrupt increases in dose due to the reducedspeed or stopping of this relative motion can be prevented, therebyprotecting the sample. An embodiment of this aspect of the presentinvention is described below, with reference to the drawings.

FIG. 57 shows an example of a multibeam inspection apparatus having adose control function incorporated therein. This multibeam inspectionapparatus 1-13 has essentially the same configuration as that of theelectron beam inspection apparatus described above, but the system inFIG. 57 is configured to have two selectable operating modes: an‘observation mode’ in which the sample image is acquired with the stagestopped, and an ‘inspection mode’ in which the sample image is acquiredwith the stage moving at high speed. A particular feature of thismultibeam inspection system 1-13 is that it has a ‘sample protectionmechanism’ for protecting a sample on the stage if a fault conditionoccurs for one reason or another when the system is operating ininspection mode.

Like the apparatus described above, this multibeam inspection apparatus1-13 comprises a primary optical system 10-13, a secondary opticalsystem 30-13, a detector unit 40-13, and a vacuum chamber (not shown).An electron gun 11-13 is placed at the topmost position of the primaryoptical system 10-13. The electron gun 11-13 accelerates electronsemitted from its cathode by thermionic emission and converges them to befurther emitted as beams. This electron gun 11-13 has a lanthanumhexaboride (LaB₆) cathode that is machined (as shown in the inset 11a-13 of FIG. 57) to enable multiple electron beams to be drawn from it.

Connected to the electron gun 11-13 is an electron gun control unit20-13, for controlling an electron gun accelerating voltage Vac, and forturning the electron gun power supply on and off. Also provided are agun alignment mechanism and gun aligner (neither of which is shown inthe drawing) for adjusting the position, etc. of the electron gun 11-13.

Also, placed along an electron beam optical axis A, is a two-stage lenscomprising the electrostatic lenses 12-13 and 15-13, a multi-apertureplate 13-13, and a primary deflector 16-13. Formed in the multi-apertureplate 13-13 are a plurality of apertures arranged in a straight line,for forming the electron beams emitted from the electron gun 11-13 intoa plurality of primary beamlets.

The electrostatic lenses 12-13 and 15-13 of the primary optical system10-13 are both rotation-symmetrical 3-electrode or 2-electrodeelectrostatic lenses. By optimizing the voltages applied to theselenses, the primary beamlets can be re-formed into beams having thedesired dimensions without loss of emitted electrons. The lens voltagesapplied to the electrostatic lenses are controlled by a primary opticalsystem control unit 21-13 connected to the primary optical system 10-13.

The primary deflector 16-13 may be either an electrostatic orelectromagnetic deflector. For example, if the primary deflector 16-13is configured as an 8-electrode electrostatic deflector, the paths ofthe primary beamlets can be deflected in the X direction by changing thevoltage applied to opposite electrodes along the X axis; or in the Ydirection by changing the voltage applied to opposite electrodes alongthe Y axis. The voltages applied to the electrodes of the primarydeflector 16-3 are controlled by a primary deflector control unit 22-13connected thereto.

The electron gun control unit 20-13, the primary optical system controlunit 21-13, and the primary deflector control unit 22-13 are allconnected to a host computer 23-13.

A stage 80-13 that is capable being moved in the X and Y directions witha sample L loaded thereon is also installed. A prescribed retardingvoltage Vr (to be discussed later) is applied to the stage. A stagecontrol unit 24-13 is connected to the stage 80-13. This stage controlunit 24-13 drives the stage 80-13 in the X and Y directions, and reads(at a data rate, for example, of 10 Hz) the x,y position of the stage,using a laser interferometer (not shown in the drawing), and outputs anx,y position signal to the host computer 23-13. The stage control unit24-13 also detects the travel speed of the stage 80-13 based on the x,yposition readings, and outputs a speed signal to the host computer23-13.

Internal to the secondary optical system 30-13 and placed along anoptical axis B thereof, are an electrostatic objective lens 31-13, anExB separator 32-13, a second electrostatic lens 33-13, and a thirdelectrostatic lens 34-13. The electrostatic objective lens 31-13 mightcomprise, for example, three electrostatic electrodes (none of which areshown in the drawing), with prescribed voltages applied to the first andsecond poles from the bottom of the electrostatic objective lens 31-13(i.e. the sample L end), and with the third electrode set to 0potential. The configuration of such electrostatic objective lenses iswell known to those skilled in the art.

The ExB separator 32-13 is a deflector that functions as anelectromagnetic prism, wherein only charged particles that satisfy theWien condition (E=vB, where v is the charged particle velocity, E is theelectric field, B is the magnetic field, and E ⊥ B) (e.g. primary beamelectrons) are passed through the separator in a no deflection, and thetrajectories of all other charged particles (e.g., secondary electrons)are deflected.

The second and third electrostatic lenses (33-13 and 33-14) are bothrotation-symmetrical lenses of the type referred to as unipotential orEinzel lenses, each with three poles. The operation of theseelectrostatic lenses is normally controlled by placing two outer polesat zero potential, and varying the voltage applied to a center pole.

The lens voltages of the electrostatic objective lens 31-13, the secondelectrostatic lens 33-13, and the third electrostatic lens 34-13; andthe magnetic field of the ExB separator 32-13, are controlled by asecondary optical system control unit 25-13 connected to the secondaryoptical system 30-13.

The detector unit 40-13 comprises a multi-aperture plate 41-13, and aplurality of detectors 42-13. The multi-aperture plate 41-13 is placedin the image plane of the third electrostatic lens 34-13, withrestrictions to prevent intermixing of secondary electrons from adjacentprimary beamlets. Formed in the multi-aperture plate 41-13 are aplurality of apertures equal in number to, and arranged in a straightline matching that of, the apertures of the multi-aperture plate 133.

A detector 42-13 comprises a phosphor for converting electrons to light,and a photomultiplier tube (PMT) for converting light to an electricalsignal. A strong electric field established between the multi-apertureplate 41-13 and the detectors 42-13 creates a convex lens effect nearthe apertures of the multi-aperture plate 41-13 to cause all secondaryelectrons approaching the apertures pass through the apertures. Thedetectors 42-13 are connected to the image processor unit 43-13.

The secondary optical system control unit 25-13 and the image processorunit 43-13 are connect to the host computer 23-13. A computer monitor26-13 is also connected to the host computer 23-13.

Next, the primary beams and the secondary electron trajectories, etc. ofa multibeam inspection apparatus 1-13 configured as described above willbe described in sequence.

(Primary Beams)

The beam current at which the primary beam is emitted from the electrongun 11-13 is a function of the electron gun acceleration voltage Vac.(Hereinafter, the beam current of the primary beam as it is emitted fromthe electron gun 11-13 will be referred to as ‘electron gun beam currentIa.’) The primary beamlets from the electron gun 11-13, pass through,and are influenced by the lens action of, the primary optical system10-13, to arrive at the primary deflector 16-13. When no voltage isapplied to the electrode of the primary deflector 16-13, the primarybeamlets are not influenced by the deflector action of that deflector;therefore, they pass through the primary deflector 16-13 to be injectedinto the center of the ExB separator 32-13. Next, the primary beamletspass through the electrostatic objective lens 31-13 to effect multibeamirradiation of the sample L.

The beam current of the primary beamlets irradiating the sample L(hereinafter ‘irradiation beam current Ib.’), however, is lower, by far,than the above electron gun beam current Ia. However, since we havepre-knowledge of the relationship between the irradiation beam currentIb and the electron gun beam current Ia, and pre-knowledge of therelationship between electron gun beam current Ia and acceleratingvoltage Vac, then we also have pre-knowledge of the relationship betweenirradiation beam current Ib and accelerating voltage Vac.

Accordingly, the primary beam irradiation current Ib can be set to thedesired value by controlling the electron gun 11-13 accelerating voltageVac, through the electron gun control unit 20-13. The electron guncontrol unit 20-13 outputs information related to the irradiation beamcurrent Ib settings to the host computer 23-13. As shown in Table 1, theirradiation beam current Ib is different in the observation andinspection modes (to be discussed later).

TABLE 1 Observation Mode Inspection Mode Primary Beam 62.5 nA 250 nAIrradiation Current Ib

The shape of the region on the sample L that is irradiated by theprimary beams, on the other hand, can be adjusted to the desireddimensions by controlling the primary optical system 10-13 lensvoltages. The irradiation beam current Ib can be adjusted for uniformirradiation of the sample L between primary beamlets.

The relationship between dose Do, the total surface area S irradiated bythe primary beamlets when the stage 80-13 is stopped (as it is inobservation mode), and the irradiation time T of the primary beamlets,can be expressed by equation (1):Do∝Ib×T/S   (1)

Thus dose Do is proportional to irradiation beam current Ib andirradiation time T.

Also, the relationship between the dose Dv when the stage 80-13 is inmotion (as it is in inspection mode, for example), and the speed ofmotion V of the stage 80-13 (V≠0) can be expressed by equation (2):Dv∝Ib/V/S   (2)

Thus Dv is directly proportional to irradiation beam current Ib, andinversely proportional to speed of stage motion V.

There is a limit, however, to the amount of radiation that can betolerated by the sample L, and if the sample continues to be irradiatedafter the maximum allowable radiation dose has been exceeded, the samplemay become contaminated or charged, and in the worst case, could bedestroyed. For this reason, data on the permissible dose range for eachtype of sample L is determined in advance and stored in the memory ofthe host computer 23-13. This permissible dose range data is then usedby a sample protection mechanism to be described later.

The voltage applied to the primary deflector 16-13 is changed to deflectthe primary beams as required to move the beams over the x,y positionsof the irradiation regions of the sample L.

(Secondary Beams)

When a sample L is irradiated by primary electron beams, electron beams(hereinafter referred to as secondary beams) are caused to be emittedfrom the irradiated region of the sample L. These secondary beams aremade up of at least one of three classes of electrons: secondaryelectrons, reflected electrons, and backscattered electrons. Thesesecondary beams possess two-dimensional image information on theirradiated region. Also, because the primary beams radiate perpendicularto the surface of the sample, as described above, the secondary electronimage information is clear and shadow-free.

A retarding voltage Vr is applied to the stage 80-13 on which the sampleL is loaded. This sets up an electric field (an accelerating field forthe secondary beams) between the sample L and an electrode of theelectrostatic lens 31-13, causing the secondary electron beams emittedfrom the sample L to be accelerated toward the electrostatic objectivelens 31-13.

The secondary beams are then converged by the electrostatic objectivelens 31-13, deflected by the ExB separator 32-13, and passed through thesecond electrostatic lens 33-13, to be imaged at the apertures of themulti-aperture plate 41-13. At this time, the secondary beams releasedfrom the surface of the sample by the primary beams are imaged onapertures of the multi-aperture plate 4-13 corresponding to theapertures of the multi-aperture plate 13-13. That is, each secondarybeamlet is imaged at the secondary aperture that corresponds to theprimary aperture that formed the primary beamlet that produced it.

Because the imaging of the secondary beams from the sample L isperformed by the electrostatic objective lens 31-13 in cooperation withthe second electrostatic lens 33-13, lens aberrations can be suppressed

The two-dimensional multibeam image formed at the apertures ofmulti-aperture plate 41-13 is first converted to light by the phosphorelements of the detectors 42-13, and then converted to an electricalsignal by the PMTs.

At this point, correspondence between the terminology used in thedescription of this embodiment and that used the related claims will beshown. That is,

-   -   the ‘motion device’ of the claims corresponds to the stage        80-13, and the stage control unit 24-13 of the present        embodiment;    -   the ‘measurement device’ of the claims corresponds to the        electron gun control unit 20-13, the stage control unit 24-13,        and the host computer 23-13 of the present embodiment;    -   the ‘decision device’ of the claims corresponds to the host        computer 23-13 of the present embodiment; and    -   the ‘control device’ of the claims corresponds to the primary        deflector 16-13, the primary deflector control unit 22-13, and        the host computer 23-13 of the present embodiment.

Next, the operation of a multibeam inspection apparatus 1-13 configuredas described above will be described. The multibeam inspection apparatus1-13 has two operating modes: an ‘observation mode,’ in which the imageof a sample L is acquired with the stage 80-13 stopped, and an‘inspection mode,’ in which the image of the sample L is acquired withthe stage 80-13 moving at high speed. In both modes, the multibeaminspection apparatus 1 is adjusted so that the beam size, on the sampleL of each beamlet will be 0.1 micron.

First, the observation mode will be described. In the observation mode,the stage control unit 24-13 drives the stage 80-13 in the X and Ydirections as required to position the region of the sample L surface tobe observed (e.g., a region having locations in which there are defects)within the irradiation region of the primary beams. Once the stage hasbeen properly positioned, it is stopped. The electron gun control unit20-13 controls the electron gun 11-13 accelerating voltage Vac asrequired to set the primary beam irradiation beam current Ib to 62.5 nA(refer to Table 1). Signals from the image processor unit 43-13 aresent, in sequence, based on an observation timing signal from the hostcomputer 23-13.

In observation mode, the image of a region of the sample L to beobserved (e.g., a region having defect locations) can be displayedconstantly on the monitor 26-13. Thus the observation mode can be usedto make various adjustments to the system while taking (and viewing)images of a prescribed test pattern. Such adjustments might includeadjustments to correct focus and aberration of the primary opticalsystem 10-13 and/or the secondary optical system 30-13, or to adjust thedetector 42-13 brightness, etc.,

Next, the operation of the system in terms of acquiring sample images inthe inspection mode will be described. In this mode, the electron guncontrol unit 20-13 controls the electron gun 11-13 accelerating voltageVac to set the primary beam irradiation beam current Ib to 62.5 nA (seeTable 1).

The image processor 43-13 supplies drive pulses based on an observationtiming signal from the host computer 43-13, causing SEM images to beformed from the primary deflector 16-13 scan signal and the detector42-13 intensity signal.

In inspection mode, because the stage 80-13 is moved at high speed assample images are being taken, consecutive images covering an entiresample, or a comparatively large portion thereof, can be acquired in ashort amount of time. In this mode, after the acquisition of sampleimages has been completed, the host computer 23-13 executes an imagedata-to-template matching process to identify defect locations on thesample L. To speed up the inspection process in this mode, the speed ofmotion of the stage 80-13 is increased, the scan signal sent to theprimary deflector 16-13 is set to a higher scan rate, and theirradiation beam current Ib of the primary beam is also increased by anamount corresponding to the increase in stage speed and transfer rate.

Thus in the inspection mode, the stage is constantly being moved at highspeed, and the sample constantly irradiated at high beam current, toachieve high-speed inspection. As stated earlier, however, if the stageshould stop or slow down for one reason or another during such highspeed scanning, the high current beams would continue to irradiate thesame spots or vicinities on the surface of the sample, causing an abruptincrease in dose (see equations (1) and (2)). A sample L, can tolerateincreased dose only within a limited range, however, and if it continuesto be irradiated after the maximum allowable radiation dose has beenexceeded, contamination or charge-up of the sample L could result, andin the worst case, the sample could be destroyed. For this reason, asample protection mechanism has been incorporated in the presentembodiment of the multibeam inspection apparatus, for protecting thesample L. This protection mechanism is described below.

Shown in FIGS. 58( a) and (b) are flow diagrams for one embodiment ofthe sample protection mechanism. As shown in FIGS. 58( a) and (b), whenthe host computer 23-13 receives an external ‘activate inspection mode’command input (S10), it acquires permissible dose range data for thesample L from memory (S11).

Next, the host computer 23-13 acquires a signal input from the stagecontrol unit 24-13 relating to the speed of motion V of the stage 80-13.Based on this speed of motion V, the primary beam irradiation beamcurrent Ib, and the area S of the irradiation region (equation (2)), thehost computer computes an actual radiation dose Dv for the sample L(S12).

Then, in Step S13, the host computer 23-13 compares the permissible dosedata acquired in Step S11 with the actual dose Dv computed in Step S12.If the actual dose Dv is less than the maximum permissible dose,execution returns to, and repeats, Step S12. Therefore, as long as theactual dose Dv is less than the maximum permissible dose, the inspectionmode sample image acquisition operation continues as described above.

On the other hand, if the actual dose Dv computed by the host computer23-13 in Step S12 exceeds the maximum permissible dose, the inspectionoperation in progress is deemed to be faulty, and a fault warning isoutput to the primary deflector control unit 22-13 (S14).

When the primary deflector control unit 22-13 receives the fault warningfrom the host computer 23-13 (S21), it applies blanking voltage to theprimary deflector 16-13, thereby deflecting the primary beams by a largeamount (thus blanking the beams) (S22). This prevents the sample L frombeing irradiated by the high-beam-current primary beam. Consequently, asituation in which the sample L would have been contaminated, or in theworst case, destroyed, has been avoided.

Moreover, although in the description of the above embodiment, anexample was described wherein a fault warning was output from the hostcomputer 23-13 to the primary deflector control unit 22-13, in order tocause the primary deflector 16-13 to blank the primary beams, thepresent invention is not limited to this method. For example, the hostcomputer 23-13 may instead be caused to output a fault warning to theelectron gun control unit 20-13, which would then turn off the powersupply for the electron gun 11, to thereby stop the emission of electronbeams from the electron gun 11-13. Also, in cases where a deflector inaddition to the primary deflector 16-13 is provided in the path of theprimary beam, the same kind of blanking control as that described abovemay be performed, using this other deflector.

In addition, in the above described embodiment of the sample protectionmechanism, the primary beams were completely shut off in response to afault warning from the host computer 23-13, thus putting the system in astate wherein the sample L was not irradiated at all by the primarybeams. Sudden large increases in dose may, however, also be prevented bycontrolling irradiation by the primary beam such that the currentdensity of the primary beam (=irradiation beam current Ib/irradiatedregion area S) is reduced. Specifically, there is a method for using theprimary deflector 16-13 to deflect the primary beams over a large areaat a rapid scan rate so that they will not remain in the same spot overa sample irradiation region. Methods that use the primary optical system10-13 to enlarge the cross-section of the primary beam in order toincrease the area S of the irradiation region, thereby reducing thecurrent density of the primary beams, can also prevent sudden largeincreases in dose. Similarly, methods wherein the electron gun 11-13acceleration voltage Vac is controlled to reduce the primary beamelectron gun beam current Ia can also prevent large increases in dose.

Moreover, although in example of the embodiment described above, theactual dose Dv was computed based on the speed of motion of the stage80-13 as detected by the stage control unit 24-13, the present inventionis not limited to this method. For example, since an increase in thedose of the sample L will be accompanied by an increase in the volume ofsecondary electrons contained in the secondary beams emitted from thesample, the volume of secondary electrons may instead be detected, andthe actual dose Dv then determined from the relationship between thequantity of secondary electrons in the secondary beams and the sample Ldose. (The quantity of secondary electrons in the secondary beams can bemeasured by periodically sampling the output of the detector 42-13.) Inaddition, since an increase in the sample L dose will also beaccompanied by brighter sample images with lower contrast ratios, thecontrast ratios of the sample images may be detected, and the actualdose Dv then determined from the relationship between this contrastratio and the sample L dose. (The contrast ratios of the sample imagescan be determined by comparing the average value of the detector pixeldensity values stored in the internal memory of the image processor unit43-13 with a threshold density value determined in advance.)

Also, in the above embodiment, data on permissible dose ranges forsamples L was stored in the memory of the host computer 23-13, and adecision as to the inspection mode fault status was then made bycomparing the actual dose Dv against this permissible dose range data.The present invention, however, is not limited to this method. Forexample, data on permissible stage motion speed ranges for the stage80-13 may instead be computed in advance based on the permissible doserange data for various samples, and these computed data stored in thememory of the host computer 23-13. In this method, the inspection modefault status decision is made by comparing the actual speed of the stage80-13 with the permissible stage motion speed range data.

Similarly, permissible ranges of secondary beam electron quantity may becomputed in advance based on sample L permissible dose data, and thispermissible secondary electron quantity data stored in the memory of thehost computer 23-13. In this method, the inspection mode fault statusdecision is made by comparing the actual secondary electron quantitywith the permissible secondary electron quantity data computed above.

In addition, permissible ranges of image data contrast ratio may becomputed in advance based on sample L permissible dose data, and thispermissible image contrast ratio range data stored in the memory of thehost computer 23-13. In this method, the inspection mode fault statusdecision is made by comparing the actual image contrast ratio with thepermissible image contrast ratio range data computed above.

Also in the above embodiment, the primary beams were not deflected(i.e., the irradiation regions were not moved), while sample images werebeing taken in the inspection mode, but it is also possible to implementthe present invention such that the primary beams are deflected (and theirradiation regions moved) while sample images are being acquired.

Moreover, the sample protection mechanism of the present invention asdescribed above can be applied to either multibeam or single beamelectron beam apparatus (including single beam SEM), as long the systemconfiguration is such that relative motion is effected between theprimary beam and the sample during acquisition of sample images.

It can be understood from the foregoing, then, that according to theabove electron beam inspection apparatus, in systems wherein relativemotion is maintained between the sample and the electron beams duringirradiation of the sample by the electron beams, abrupt increases indose due to slowing or stopping of the above relative motion can beprevented, to thereby protect the sample. This provides improvedreliability for high-beam-current, high-speed (in particular,high-scan-rate) inspection processes.

Control Elements for Deflectors, Wien Filters, Etc.

In the electron beam inspection apparatus described above, the electronoptical systems are made up of elements such as electrostaticdeflectors, electrostatic lenses, and Wien filters. FIG. 61 shows a planview of a prior electrostatic deflector 100-14. Shown in FIGS. 62( a)and 62(b), respectively, are cross-sections of sections A-A and B-B ofFIG. 61. The electrostatic deflector 100-14 has eight metal electrodes101-14, each of which is fastened to the inside wall of an insulatingouter cylinder 102-14 by fastening bolts 103-14 and 104-14 (FIG. 61 andFIG. 62( a)). Wires 105-14 for applying voltage to each of the metalelectrodes 101-14 are secured directly to the electrodes using wiringscrews 106-14 (FIG. 62( b)).

In this electrostatic deflector 100-14, an electrostatic fieldresponsive to the voltages applied to the metal electrodes 101-14 isformed within a space 107-14 enclosed by the inward facing surfaces101-14 a of the metal electrodes 101-14. Therefore, a charged particlebeam passing through the space 107-14 along a center axis Z thereof,will be deflected responsive to the electrostatic field formed therein.

Gap portions 108-14 are formed between each pair of adjacent metalelectrodes 101-14. These gap portions 108-14 are formed in a shape thatextends outward from the space, not in a straight line, but by makingtwo sharp turns before arriving at the insulating outer cylinder 102-14.This is done to avoid having a direct view of the exposed portions109-14 of the insulating outer cylinder 102-14 from a charged particlebeam passing through the space 107-14. This configuration prevents theinsulating outer cylinder 102-14 from becoming electrically charged, andthus makes it possible to effect precise control of the electrostaticfield within the space 107-14 responsive to the voltages applied to themetal electrodes 101-14.

However, there was a problem with the above electrostatic deflector101-14, in that it had a complex structure using a large number ofparts, which made cost and size reduction difficult. Also the eightmetal electrodes 101-14 that constituted the electrostatic deflector100-14 were obtained by dividing a metal cylinder into sections after ithad been bolted to the inside of the insulating outer cylinder 102-14,which made it difficult make any improvements in the accuracy of theangles at which the electrodes were divided, or the circularity of thespace 107-14 enclosed by the inside surfaces 101 a-14 of the metalelectrodes 101-14.

This led to a proposal for replacing these metal electrodes 101-14 withplated electrodes formed on an insulator by a surface process such aselectroplating. The motivation for using plated electrodes was toeliminate the electrode mounting bolts, and provide smaller deflectorswith lower parts counts.

As in the prior electrostatic deflector 100-14 described above, however,in deflectors with plated electrodes, when wires for applying voltage tothe electrodes were fastened directly to the plated electrodes withscrews, the electrodes could possibly be left with surface openings.Such openings in the electrodes distorted the electrostatic fielddistribution within the space through which the charged particle beampassed, such that deflection of the charged particle beam could not beaccurately controlled.

This led to a configuration in some electrostatic deflectors wherein asupport portion of the insulator on which the plated electrodes wereformed was caused to protrude (along with the electrodes) from the endof an insulating outer cylinder, and the wires were attached to thisprotruding portion. This eliminated the holes in the electrodes, but theconstruction related to the wiring was complex, and was such thatinsulated wire coverings could possibly be visible through the gapsbetween adjacent electrodes.

The need for charged particle beam control elements using platedelectrodes exists not only for electrostatic deflectors, but also forother elements, such as electrostatic lenses. For these other elementsas well, there is a need for a better way of connecting the voltageapplication wires to the plated electrodes.

To address the above problem, the present invention provides a chargedparticle beam control element (deflector, lens, etc.) wherein it ispossible, through a simple structural configuration, to accuratelysecure the surfaces of electrodes formed on an insulator by a surfaceprocess such as electroplating, etc., and also to connect wires to theseelectrodes for application of voltages thereto.

Shown in FIG. 59 is a simplified drawing of an embodiment of one of thecharged particle beam control elements of the present invention, as usedin a deflector or Wien filter FIG. 60 shows a cross-section of thecharged particle beam control element. In FIGS. 59 and 60, the chargedparticle beam control element has a base 1-14 made of an insulatingmaterial. The base 1-14 is a cylinder that has an axis A at its center,and a structure defined by a through-hole 5-14 that is formed by anouter surface 2-14, and end surfaces 3-14 and 3′-14. The through-hole5-14 is concentric with the axis A, which is coincident with the opticalaxis during use. Slots 6-14 dividing the electrodes are formed in thecylindrical base 1-14 in a direction parallel to the axis A andextending radially outward therefrom. As shown in the drawing, each ofthe slots 6-14 is formed with a sharp bend identical to that of theother slots, and with a circular through-hole 7-14 at its terminal end.In addition, as shown in FIG. 60, conductive shield cylinders 21-14having the same diameter as the through-hole 5-14, are placed at the topand bottom, respectively, of the base 1-14 of the charged particle beamcontrol element (i.e., near the opposite end surfaces 3-14 and 3′-14thereof).

In the structure, as described, a metallic coating is applied to the endsurfaces 3-14 and 3′-14 opposite an inner surface 4-14, except fornon-conductive surfaces 10-14 thereof, which are provided forinsulation.

Specifically, a plurality of electrodes 8-14 are formed on the innersurface 4-14, separated from each other by the slots 6-14, as indicatedby diagonal-line hatching in FIG. 59. Also formed on the respective endsurfaces 3-14 and 3′-14, are a plurality of conductors 9-14, each ofwhich is electrically connected to the electrode 8-14. A metalliccoating is also applied to the inside surfaces (also indicated bydiagonal-line hatching) of the slots 6-14, which extend outward from theelectrodes 8-14 formed in the inner surface 4-14, to the through-holes7-14. Metallic coating is not applied, however, to the inside surfacesof the through-holes 7-14, or the end surface portions 10-14 extendingoutward from the through-holes 7-14 to the outer surface 2-14. In thismanner the slots 6-14 are electrically isolated from each other by thenon-conductive surface portions 10-14, while a plurality of electricallycontinuous portions are formed from the electrically connectedelectrodes 8-14 and conductors 9-14. In the embodiment of FIG. 59, 8slots 6-14 are formed, thus there are also 8 sets of electricallyinterconnected electrodes 8-14 and conductors 9-14. If required,additional conductors connected to the above conductors 9-14 may also beformed on the outer surface 2-14.

Fine wire is used for the wires 11-14 for the electrodes 8-14, with thefine wires bonded either to the outer surface 2-14, or one of the endsurfaces 3-14 and 3′-14. If the wires 11-14 are brought out from theouter surface 2-14, it makes the outside diameter of the chargedparticle beam control element 1-14 larger, while extra space is requiredin the axis A direction of the charged particle beam control element ifthey are brought out from the end surfaces, 3-14 and 3′-14. FIG. 59shows an example in which the wires 11-14 are brought out from one ofthe end surfaces 3-14.

Moreover, in the present invention,] it is preferred that the relationL/D<4.0 be satisfied, where, (as indicated in FIG. 60),

-   -   D is the distance between the shield cylinders 21-14 in the        surfaces thereof that include the axis A and the opposing        through-holes 7, and the end surfaces 3-14 of the base 1-14; and    -   L is the radial distance between the surfaces of the electrodes        8-14 on the ends thereof nearest the axis A, and the surfaces of        the through-holes 7-14 on the sides thereof nearest the axis A.

If this condition is met, when an electrical charge develops on theinner surfaces of the through-holes 7-14 of the base 1-14, the effect ofthe electrical potential resulting from that charge on a chargedparticle beam passing through near the axis A will be less than 1/1000.

The charged particle beam control element shown in FIG. 59 can be usedas an electrostatic deflector or ExB separator/Wien filter in anelectron beam inspection apparatus as described above.

As will be understood from the above description, through the chargedparticle beam control element of the present invention, the surfaces ofelectrodes formed on an insulating surface by a surface process such aselectroplating can be accurately secured, and wires can be connected tothese electrodes for application of voltage thereto, in a simplestructural configuration, to thus contribute to size and cost reductionof charged particle beam control elements and charged particle beamapparatus, while also providing more precise control of charged particlebeam paths by the charged particle beam elements.

Semiconductor Device Fabrication Method

As is clear from the above discussion, the inspection apparatus of thepresent invention is capable of high throughput sufficient to enableperformance of appropriate inspections of samples such as waferscurrently undergoing a fabrication process, without holding up theprocess. A semiconductor device production method designed to includethe in-process performance of such inspections will be described withreference to FIGS. 63 and 64.

FIG. 63 shows a flow chart for one embodiment of the semiconductordevice production method of the present invention. This embodimentcomprises the following steps:

-   (1) A wafer fabrication step for fabricating a wafer (or a wafer    preparation step for preparing a wafer).-   (2) A mask fabrication step for fabricating a mask (or a mask    preparation step for preparing a mask) to be used for exposures.-   (3) A wafer processing step for performing required machining    (etching) processes on a wafer.-   (4) A chip assembly step for dicing (cutting out) individual chips    from the multiple chips formed on a wafer and assembling them to an    operational state.-   (5) A chip inspection step for inspecting the completed chips.

Each of the above main steps comprises a number of sub steps. Withinthese main steps, one main step that has a decisive influence on theperformance of the semiconductor devices produced is Step (3) the waferprocessing step. In this step, patterns for the designed circuit areformed in sequential layers, one on top of the other (a large number ofsuch layers for a device that will operate as a memory or MPU).Therefore the wafer processing step further comprises the followingsteps:

-   (1) A membrane forming step (using CVD, sputtering, etc.), for    forming dielectric films for insulating layers, and metal thin films    for wiring and electrodes.-   (2) An oxidation step, for oxidizing these thin film layers and    wafer substrates.-   (3) A lithography step using a mask (reticle), for selectively    etching the membrane layers, wafer substrates, etc. to form resist    patterns.-   (4) An etching step (using dry etch technology, etc.), for machining    the thin films or substrates to conform to the resist pattern.-   (5) An ion implantation/impurity diffusion step.-   (6) A resist stripping step.-   (7) An inspection step, for inspecting processed wafers.

Moreover, these wafer processing steps are repeated for each layer asrequired to manufacture semiconductor devices what will operate asdesigned.

FIG. 64 is a flow chart for the lithography step, which forms thenucleus of the wafer processing step of FIG. 63. The lithography stepincludes the following steps:

-   (1) A resist application step, for applying a coat of resist to a    wafer that had a circuit pattern formed on it in preceding steps.-   (2) An exposure step, for exposing the resist.-   (3) A developing step, for developing the exposed resist to obtain a    resist pattern.-   (5) An annealing step, for stabilizing the developed resist pattern.

The above semiconductor device fabrication steps (wafer processing step,lithography step, etc.) are well known, and should require no furtherexplanation.

In the above step (7), the wafer inspection step, if the defectinspection methods and defect inspection apparatus of the presentinvention are used, it will be possible to perform high-throughputinspection even of semiconductor devices having extremely fine patterns.This will make inspection of 100% of manufactured devices feasible,which will improve product yield and prevent defective devices frombeing shipped.

A number of embodiments of the invention were described above, but thepresent invention is not limited to these embodiments, and manyvariations thereof may be made without deviating from the scope of theinvention.

1. An electron bean apparatus comprising a plurality of optical systems,each of which comprises: an electron beam irradiating means, forirradiating a sample with primary electron beams, and scanning thesample by deflecting an incident angle of the primary electron beamswithin a prescribed range; a deflecting/separating means for deflectingsecondary electron beams, which are generated by irradiation of theprimary electron beams onto the sample, in a prescribed direction withrespect to an optical axis of the electron beam irradiating means; and adetecting means for detecting the secondary electron beams deflected bythe deflecting/separating means, wherein: said optical systems arearranged in two rows, such that paths of the secondary electron beamsdeflected by the deflecting/separating means do not interfere with eachother; said optical systems are each capable of evaluating the sample byscanning different regions on the sample with their primary electronbeams and detecting the respective secondary electron beams emitted fromeach of the respective regions.
 2. An electron beam apparatus as recitedin claim 1, wherein: in each of the plurality of optical systems, theelectron beam irradiating means irradiates a plurality of primaryelectron beams on the sample; and the detecting means uses a pluralityof detector elements, which are separated from each other, to detect theplurality of secondary electron beams generated by irradiation of theplurality of primary electron beams.
 3. An electron beam apparatus asrecited in claim 2, wherein the plurality of optical systems arearranged in a single row, and the paths of the secondary electron beamsare arranged substantially perpendicular to a direction of the row ofthe optical systems; and evaluation of the sample is enabled by movementof the sample in a direction substantially perpendicular to thedirection of the row.
 4. An electron beam apparatus as recited in claim3, wherein the evaluation of the sample includes, at least, any one ofan inspection for any defect of the sample, a measurement of a linewidth of a pattern formed on the sample, a measurement of electricalpotential of the pattern, and a measurement of movement of the sample byirradiation of primary electron beams having a short pulse duration. 5.A charged particle beam control element for controlling a chargedparticle beam with an electrostatic field, the charged particle beamcontrol element comprising: a base made from a cylindrical insulator andhaving a through-hole therein; a plurality of electrodes that areelectrically insulated from each other and that are formed by a surfaceprocess performed on an inner surface of the base; a plurality ofconductors formed by said surface process on at least an end surface ofthe base, such that they are electrically connected to the respectiveelectrodes; and wires connected to the respective conductors.
 6. Acharged particle beam control element as recited in claim 5, wherein thebase has slots that electrically isolate the electrodes and conductorsfrom each other, whereby film-less surfaces for insulating theconductors from each other are provided in the surface on which theplurality of conductors are formed.
 7. A charged particle beam elementas recited in claim 6, wherein a shielding conductor is placed aprescribed distance away from said end surface, with said prescribeddistance being less than the distance between said film-less surfaceportions of the slots and the electrode surfaces.